Revert "utf8n_to_uvuni: Use utf8_to_uv()"

This reverts commit d62b9fa.
That commit turns out to be ill advised.  I forgot that the function
call it replaced also calls utf8_to_uv() and then munges its return value.
This commit to succeed would have needed the same munging.  But rather
than repeat that logic, just call the original function, by simply
reverting this commit.

This fixes Perl#23053.  I haven't tested them, but it likely also takes care
of Perl#22820, Perl#23086, and Perl#22977

Author: khwilliamson

mathoms.c

@@ -199,9 +199,7 @@ Perl_utf8n_to_uvuni(pTHX_ const U8 *s, STRLEN curlen, STRLEN *retlen, U32 flags)
 {
     PERL_ARGS_ASSERT_UTF8N_TO_UVUNI;
 
-    UV cp;
-    (void) utf8_to_uv_flags(s, s + curlen, &cp, retlen, flags);
-    return NATIVE_TO_UNI(cp);
+    return NATIVE_TO_UNI(utf8n_to_uvchr(s, curlen, retlen, flags));
 }
 
 UV