regexp.h: remove completely redundant return statement

regexp.h: remove completely redundant return statement

Remove a redundant return() statement at the end of the
get_regex_charset_name function. The "default" case for the above
switch statement will always return for us.

This was added intentionally in v5.14.0-354-g0984e55 by Jim Cromie,
but the rationale for doing so is that we might have a compiler bug
here, but we're pretty screwed anyway if switch statements stop
working as advertised by the standard so there's no reason to be
defensive in this particular case.

This is also causing a lot of whine from Sun Studio 12 Update 1:

    regexp.h", line 329: warning: statement not reached