Remove install step in ubuntu.yml (#402)

Summary:
It's used to generate docs. Since we no longer generate docs per commit
(71f9c1a), we can also remove this to
save some time on CI test.

Pull Request resolved: #402

Reviewed By: bearzx

Differential Revision: D37428114

Pulled By: wenleix

fbshipit-source-id: 285f9f1d1c3fd98f22933a4fa4fc0ec98d236f46

Author: wenleix

.github/workflows/ubuntu.yml

@@ -73,7 +73,3 @@ jobs:
 
       - name : Run TorchArrow unit tests
         run: pytest --no-header -v torcharrow/test
-
-      - name : Install TorchArrow
-        run: |
-          CCACHE_DIR=$GITHUB_WORKSPACE/.ccache python setup.py install --user