util: remove PyString* aliases on Python 3
We no longer have any users of the legacy PyString* functions. We no
longer need these redefinitions.
After this change, the only reference to "PyString" in the repo is in
watchman's C extension. That isn't our code and porting Mercurial
extensions to Python 3 is not a high priority at the moment. watchman's
C extension will be dealt with later.