cc-wrapper: drop no-longer-necessary hack

The commit prior to this one, "gcc: fix c++ headers when same
triplet cross compiling" causes gcc's c++ headers to be in the same
outpath subdirectory regardless of whether the gcc build is a
host==target or host!=target compiler.

As a result of that change, the hack in cc-wrapper which adapted to
the different paths is no longer needed.  And, in fact, it must be
removed, since if it is left in place builds such as
pkgsCross.aarch64-multiplatform.firefox will fail as shown below.

```
firefox-unwrapped>  0:02.01(B checking the host C compiler works... yes(B(B
firefox-unwrapped>  0:02.01(B checking for the host C++ compiler... /nix/store/1asqji9djmdlapzs70q7jw2j308ry7cn-clang-wrapper-16.0.6/bin/c++(B(B
firefox-unwrapped>  0:02.14(B checking whether the host C++ compiler can be used... yes(B(B
firefox-unwrapped>  0:02.14(B checking the host C++ compiler version... 16.0.6(B(B
firefox-unwrapped>  0:02.21(B checking the host C++ compiler works... yes(B(B
firefox-unwrapped>  0:02.40(B checking for target linker... lld(B(B
firefox-unwrapped>  0:02.51(B checking for host linker... lld(B(B
firefox-unwrapped>  0:02.60(B checking for 64-bit OS... yes(B(B
firefox-unwrapped>  0:02.67(B checking for new enough STL headers from libstdc++...(B(B
firefox-unwrapped>  0:02.67(B DEBUG: <truncated - see config.log for full output>(B(B
firefox-unwrapped>  0:02.67(B DEBUG: |                 #if defined(__GLIBCXX__) && !defined(_GLIBCXX_RELEASE)(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | #  error libstdc++ not new enough(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | #endif(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | #if defined(_GLIBCXX_RELEASE)(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | #  if _GLIBCXX_RELEASE < 8(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | #    error libstdc++ not new enough(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | #  else(B(B
firefox-unwrapped>  0:02.67(B DEBUG: |      (void) 0(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | #  endif(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | #endif(B(B
firefox-unwrapped>  0:02.67(B DEBUG: |                   ;(B(B
firefox-unwrapped>  0:02.67(B DEBUG: |                   return 0;(B(B
firefox-unwrapped>  0:02.67(B DEBUG: |                 }(B(B
firefox-unwrapped>  0:02.67(B DEBUG: Executing: `/nix/store/7v4bi4q334yircaznwm353h1l5i7k98f-aarch64-unknown-linux-gnu-clang-wrapper-16.0.6/bin/aarch64-unknown-linux-gnu-clang++ /build/conftest.qvbo58dv.cpp -c`(B(B
firefox-unwrapped>  0:02.67(B DEBUG: The command returned non-zero exit status 1.(B(B
firefox-unwrapped>  0:02.67(B DEBUG: Its error output was:(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | /build/conftest.qvbo58dv.cpp:1:10: fatal error: 'cstddef' file not found(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | #include <cstddef>(B(B
firefox-unwrapped>  0:02.67(B DEBUG: |          ^~~~~~~~~(B(B
firefox-unwrapped>  0:02.67(B DEBUG: | 1 error generated.(B(B
firefox-unwrapped>  0:02.67(B ERROR: The libstdc++ in use is not new enough.  Please run ./mach bootstrap to update your compiler, or update your system libstdc++ installation.(B(B
firefox-unwrapped> *** Fix above errors and then restart with "./mach build"
error: build of '/nix/store/5in7xkji5hzqkl14ygwq3vxnni54lykk-firefox-unwrapped-aarch64-unknown-linux-gnu-119.0.1.drv' on 'ssh://[email protected]' failed: builder for '/nix/store/5in7xkji5hzqkl14ygwq3vxnni54lykk-firefox-unwrapped-aarch64-unknown-linux-gnu-119.0.1.drv' failed with exit code 1
error: builder for '/nix/store/5in7xkji5hzqkl14ygwq3vxnni54lykk-firefox-unwrapped-aarch64-unknown-linux-gnu-119.0.1.drv' failed with exit code 1;
```

pkgs/build-support/cc-wrapper/default.nix

@@ -518,10 +518,10 @@ stdenv.mkDerivation {
     # additional -isystem flags will confuse gfortran (see
     # https://github.com/NixOS/nixpkgs/pull/209870#issuecomment-1500550903)
     + optionalString (libcxx == null && isClang && (useGccForLibs && gccForLibs.langCC or false)) ''
-      for dir in ${gccForLibs}${lib.optionalString (hostPlatform != targetPlatform) "/${targetPlatform.config}"}/include/c++/*; do
+      for dir in ${gccForLibs}/include/c++/*; do
         echo "-isystem $dir" >> $out/nix-support/libcxx-cxxflags
       done
-      for dir in ${gccForLibs}${lib.optionalString (hostPlatform != targetPlatform) "/${targetPlatform.config}"}/include/c++/*/${targetPlatform.config}; do
+      for dir in ${gccForLibs}/include/c++/*/${targetPlatform.config}; do
         echo "-isystem $dir" >> $out/nix-support/libcxx-cxxflags
       done
     ''