regen/mk_PL_charclass.pl: Use mktables table for charname

regen/mk_PL_charclass.pl: Use mktables table for charname

This commit uses the mktables defined table for whether or not a
character is a legitimate charname continuation.  This will allow it to
be kept in sync with other code that needs the definition.

The only change this makes is to delete "colon" from being a legitimate
continuation character.  A colon was only accepted because it was used
in the paradigm for like "Greek: Alpha", and is not part of any
actual character name.