ci: Travis: remove clang-tsan from allowed failures [skip ci] (#10591)

While it still might be flaky sometimes, it is far better than the
osx jobs in general, and due to it being allowed to fail, we are not
getting aware of more recent (flaky) issues due to building tags during
make-install, which might indicate a more generic problem.

.travis.yml

@@ -154,8 +154,6 @@ jobs:
       env:
         - CLANG_SANITIZER=TSAN
         - *common-job-env
-  allow_failures:
-    - name: clang-tsan
   fast_finish: true
 
 before_install: ci/before_install.sh