Remove a workaround for #23871 which manipulated the module
to prevent classes defined in the text editor from having their
name-space cleared.
This caused the "multiprocessing" module to store the "__main__" module
as "__mp_main__" for later use.
Accessing attributes from this module would then attempt to read from
with a null "mp_dict" which crashed. This happened when showing the
extensions preferences but would have occurred if "__mp_main__" was
accessed from elsewhere too.
Resolve by removing the workaround since it has not been needed
since Python 3.2.
Campbell Barton
d7d42275fa