end the pp_mapstart trickery

Current codebase wires Perl_pp_mapstart to Perl_unimplemented_op (by
regen/opcode.pl) [1], but then avoids runtime panics by Perl_ck_grep
changing all OP_MAPSTART nodes to use PL_ppaddr[OP_GREPSTART] [2].

This is all too clever by half, so this patch undoes the trickery, and
treats these 2 OPS like 93bad3fd5548 did for OP_AELEMFAST and \1_LEX.

I cant glean a reason for this historical arrangement:
Looking at regen/opcode.pl blamelog..

65bca31a68 added Perl_unimplemented_op() used by 3 'unreachable' ops,
and replaced a 'panic: mapstart' diag with a common one, so the trick
goes further back.

c78ff9799bf moved a minimal/DIEing pp_mapstart implementation to
mathoms.c from pp_ctl.c.  Perl_ck_grep also did the GREPSTART patching
back then.

f54cb97a39f did minor tweaks to a full pp_mapstart implementation.  I
couldnt find the commit between it and c78ff that changed pp_mapstart
to a DIEing one, or I fat-fingered it, or I got distracted.

looking at ck-grep(), the code doing [2] is from 22c35a8c23 in 1998.

So anyway, I tried the following, it worked, it seems the historical
reason is no longer relevant.

[1] change regen/opcode.pl generated mapping
   -#define Perl_pp_mapstart Perl_unimplemented_op
   +#define Perl_pp_mapstart Perl_pp_grepstart

   this sets PL_ppaddr[OP_MAPSTART] = PL_ppaddr[OP_GREPSTART] during
   init, which makes the optype trickery in ck_grep[2] unneeded.

[2] Drop re-type-ing of MAPSTARTs as GREPSTARTs by Perl_ck_grep(OP* o)
   Given 1, mapstart & grepstart share code, so just leave optype alone.

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