Delete bugbear from Python 2 lint. (pytorch#18192)

Summary:
Pull Request resolved: pytorch#18192
ghimport-source-id: 9523a09

Stack from [ghstack](https://github.com/ezyang/ghstack):
* **pytorch#18192 Delete bugbear from Python 2 lint.**

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Differential Revision: D14529240

fbshipit-source-id: 1a433b53dd38d1c455e8c0750d97c594ac51ef09

.travis.yml

@@ -23,7 +23,7 @@ matrix:
         script: cd .circleci && ./ensure-consistency.py
       - name: "Python 2.7 Lint"
         python: "2.7"
-        install: pip install flake8 flake8-bugbear
+        install: pip install flake8
         script: flake8
       - name: "Python 3.7 Lint"
         python: "3.7"