stop callers of rex engine using RX_MATCH_UTF8_set
The way that the regex engine knows that the match string is utf8 is
currently a complete mess. It's partially signalled by the utf8 flag of
the passed SV, but also by the RXf_MATCH_UTF8 flag in the regex itself,
and the value of PL_reg_match_utf8.
Currently all the callers of the engine (such as pp_match, pp_split etc)
initially use RX_MATCH_UTF8_set() before calling the engine. This sets both
the RXf_MATCH_UTF8 flag on the regex, and PL_reg_match_utf8.
Then the two entry points to the engine (regexec_flags() and
re_intuit_start()) initially repeat the RX_MATCH_UTF8_set()
themselves.
Remove the usage of RX_MATCH_UTF8_set() by the callers of the engine,
and instead just rely on the engine to do it.
Also, remove the "secret" setting of PL_reg_match_utf8 by
RX_MATCH_UTF8_set(), and do it explicitly.
This is a prelude to eliminating PL_reg_match_utf8.