perlapi: Remove newly obsolete statement

Since commit 010ab96b9b802bbf77168b5af384569e053cdb63, this function is
now longer a wrapper, so shouldn't be described as such.
regcomp.c: Don't use mathoms function

utf8n_to_uvuni() is on its way out.  Instead, this code knows that the
UTF-8 it is looking at is valid (having been already checked earlier in
the regex compilation process), so use valid_utf8_to_uvchr().  This
allows us to get rid of a flag variable.
Move functions prematurely placed into mathoms back to utf8.c

These functions are still called by some CPAN-upstream modules, so can't
go into mathoms until those are fixed.  There are other changes needed
in these modules, so I'm deferring sending patching to their maintainers
until I know all the necessary changes.