Remove use of SAVEDESTRUCTOR_X(clear_re...) from regcomp.c

Remove use of SAVEDESTRUCTOR_X(clear_re...) from regcomp.c

The purpose of this is to free the regexp when croaking.
SAVEDESTRUCTOR_X calls clear_re on scope exit.  clear_re does
ReREFCNT_dec.

It used to be necessary to do it this way before regular expres-
sions were SVs.

Since v5.10.0-145-g84679df, regexps have been SVs, and ReREFCNT_dec is
effectively an alias for SvREFCNT_dec.

There will be one less function call on scope exit when regexp compi-
lation croaks if we use SAVEFREESV instead of SAVEDESTRUCTOR_X.