remove outer (pants_exe) lock and serialized cmd

All two subcalsses of command (eg GoalRunner and SetupPy) do not override `serialized` to return `True` and thus do not require the outer lock.
Removing it simplifies pants_exe and reduces the number of places we depend on dirutil.Lock and shutil.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/41782185

Bugs closed: 817

Reviewed at https://rbcommons.com/s/twitter/r/1388/
Sync of https://github.com/pantsbuild/pants/commit/d59ea8e8ed01be36ceab5ebfd1d4079fdd2a60c5