ci: Remove x86_64-gnu-incremental builder

This builder is starting to time out frequently causing PRs to bounce and
otherwise doesn't seem to be catching too many bugs, so this commit removes it
entirely. We've had a number of timeouts in the last few weeks related to this
builder:

* https://travis-ci.org/rust-lang/rust/jobs/360947582
* https://travis-ci.org/rust-lang/rust/jobs/360464190
* https://travis-ci.org/rust-lang/rust/jobs/359946975
* https://travis-ci.org/rust-lang/rust/jobs/361213241
* https://travis-ci.org/rust-lang/rust/jobs/362346279
* https://travis-ci.org/rust-lang/rust/jobs/362072331

On a good run this builder takes about 2h15m, which is already too long for
Travis and the variable build times end up pushing it beyond the 3h limit
occasionally.

The timeouts here are somewhat expected in that an incrementally compiled rustc
compiler isn't optimized like a normal rustc, disallowing inlining between
codegen units and losing lots of optimization opportunities.

.travis.yml

@@ -176,8 +176,6 @@ matrix:
       if: branch = auto
     - env: IMAGE=x86_64-gnu-distcheck
       if: branch = auto
-    - env: IMAGE=x86_64-gnu-incremental
-      if: branch = auto
 
     - stage: publish toolstate
       if: branch = master AND type = push