sv.c: Remove redundant sv_force_normal calls from sv_2[iun]v
The previous commit made it possible to nummify a string whose SvLEN
is 0. So we don’t need to run shared hash key scalars through
sv_force_normal before nummifying them. We still need to run COW sca-
lars through sv_force_normal under PERL_OLD_COPY_ON_WRITE, as it uses
the IVX field for COW bookkeeping. For simplicity’s sake, I’m not
bothering to distinguish shared hash keys scalars from other COW sca-
lars under PERL_OLD_COPY_ON_WRITE.