regcomp.c: Generate EXACTFU_SS only for non-UTF8

regcomp.c: Generate EXACTFU_SS only for non-UTF8

It turns out that now, the regular methods for handling multi-character
folds work for the ones involving LATIN SMALL LETTER SHARP S when the
pattern is in UTF-8.  So the special code for handling this case can be
removed, and a regular EXACTFU node is generated.  This has the
advantage of being trie-able, and requiring fewer operations at run
time, as the pattern is pre-folded at compile time, and doesn't have to
be re-folded during each backtracking at run-time.

This means that the EXACTFU_SS node type will only be generated for
non-UTF-8 patterns, and the handling of it is unchanged in these cases.