Drop environment variable no longer needed after parent commit

Author: jtgeibel

.travis.yml

@@ -12,7 +12,6 @@ env:
     - JOBS=1 # See https://git.io/vdao3 for details.
     - DATABASE_URL=postgres:///cargo_registry_test
     - TEST_DATABASE_URL=postgres:///cargo_registry_test
-    - CARGO_TARGET_DIR=target
     - PERCY_PARALLEL_TOTAL=2
     # Percy secrets are included here to enable Percy's GitHub integration
     # on community-submitted PRs