Gut Perl_save_re_context

What it does is not longer necessary.  See ticket #122747 and
commits 2c1f00b90 and 7d75537e.

The only CPAN module using this function, XML::Fast, passes its tests
with this change.
Don’t call save_re_context

It is an empty function.
Remove obsolete comment from utf8.c

The call to save_re_context was removed by the previous commit.  The
commit before that stopped save_re_context from doing anything.

Commit db2c6cb33 stopped the errsv_save line from triggering
get-magic.

So this comment, added in dc0c6abb4, no longer applies.
utf8.c: Move an #ifndef for clarity

The comment really belongs inside it, as it refers to those two
lines of code.