fix for clang-tidy (pytorch#16164)

Summary:
It turns out that clang-tidy is bundled with travis's standard trusty distribution, so no need to install it manually.
Pull Request resolved: pytorch#16164

Differential Revision: D13738986

Pulled By: suo

fbshipit-source-id: d0cd76c615625b2ed7f18951289412989f15849d

Author: suo

.travis.yml

@@ -34,11 +34,5 @@ matrix:
         script: cd docs/cpp/source && ./check-doxygen.sh
       - env: CLANG_TIDY
         python: "3.6"
-        addons:
-          apt:
-            sources:
-              - ubuntu-toolchain-r-test
-              - llvm-toolchain-trusty
-            packages: clang-tidy
         script: tools/run-clang-tidy-in-ci.sh