Kill confusing / obsoleted comment.

As suggested in https://rbcommons.com/s/twitter/r/1660

Testing Done:
CI went green here: https://travis-ci.org/pantsbuild/pants/builds/48221855

Bugs closed: 991

Reviewed at https://rbcommons.com/s/twitter/r/1664/

src/python/pants/option/parser.py

@@ -146,7 +146,6 @@ def register(self, *args, **kwargs):
     self._validate(args, kwargs)
     dest = self._set_dest(args, kwargs)
 
-    # Is this a boolean flag?
     inverse_args = []
     help_args = []
     for flag in self.expand_flags(*args, **kwargs):