Remove the old setup_py command.

It has been replaced by the python_setup goal.

There are no longer any remaining commands in pants, other than GoalRunner.

Note that this requires 49bb9f43eeb1331342f35cef331265cf4b8199bb, so it may not work if you cherry-pick it.

Testing Done:
CI passes.

Reviewed at https://rbcommons.com/s/twitter/r/1448/
Move the spec exclude integration test from command to base.

It is base-level functionality that has nothing to do with command.

Testing Done:
Integration test passed in its new location.

CI baking.

Reviewed at https://rbcommons.com/s/twitter/r/1449/
Sync of https://github.com/pantsbuild/pants/commit/7a0d79a5996c51633a9a36ce8eed5e5881a2eaeb