travis: do not allow CPU 4 test failures

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

.travis.yml

@@ -27,8 +27,6 @@ env:
 matrix:
   fast_finish: true
   allow_failures:
-  - go: 1.12.7
-    env: TARGET=linux-amd64-integration-4-cpu
   - go: 1.12.7
     env: TARGET=linux-amd64-grpcproxy
   - go: 1.12.7