regexec_flags(): simplify length calculation

regexec_flags(): simplify length calculation

The code to calculate the length of the string to copy was

    PL_regeol - startpos + (stringarg - strbeg);

This is a hangover from the original (perl 3) regexp implementation
that under //i, copied and folded the original buffer: so startpos might
not equal stringarg. These days it always is (except under a match failure
with (*COMMIT), and the code we're interested is only executed on success).

So simplify to just PL_regeol - strbeg.