Don’t localise CopSTASH(&PL_compiling) in newCONSTSUB

Don’t localise CopSTASH(&PL_compiling) in newCONSTSUB

When newCONSTSUB was added in commit 5476c433, it did not set
curcop temporarily to &compiling, and so gv_fetchpv would look at
curcop->cop_stash.  So cop_stash needed to be localised.

(Time passes.... curcop is now PL_curcop.  &compiling is
now &PL_compiling.  gv_fetchpv is now gv_fetchpvn_flags.
curcop->cop_stash is now CopSTASH(PL_curcop).)

Since commit 401667e9, newCONSTSUB has set PL_curcop to &PL_compiling
temporarily.  When that is the case, gv_fetchpvn_flags uses
PL_curstash and ignores CopSTASH(PL_curcop).

So this localisation is no longer necessary, as newCONSTSUB has always
set (PL_)curstash.