gui: remove obsolete hbox for gtk3/qt5/qt6 #3458

Job	Run time
linux-compiler-compatible (clang, Debug, libc++)	12m 44s
mac-compiler-compatible (macos-14, Debug)	11m 3s
linux-compiler-compatible (clang, Release, libc++)	9m 45s
mac-compiler-compatible (macos-14, Release)	9m 48s
mingw64-compiler-compatible-crosscompiling (i686, Debug)	15m 43s
mingw64-compiler-compatible-crosscompiling (i686, Release)	16m 37s
mingw64-compiler-compatible-crosscompiling (x86_64, Debug)	15m 20s
mingw64-compiler-compatible-crosscompiling (x86_64, Release)	15m 14s
win-compiler-compatible-2022 (x86, dynamic, Debug)	35m 1s
win-compiler-compatible-2022 (x86, dynamic, Release)	31m 14s
	2h 52m 29s

Provide feedback