remove almost unreachable NULL sv arg code from sv_2*n_flags

The NULL sv code being removed dates to commit e334a159a5 Perl 1.0 as
the pre-SV str_2ptr and str_2num calls. When SVs were intoduced in
commit 79072805bf Perl 5.0 alpha 2, the NULL sv code was copied to the new
SV functions. The functions were bulk marked non-NULL in commit f54cb97a39
during 5.9.3 development. The docs were corrected to say NULLOK support
in commit 53e8571218 during 5.11.0.

See the perldelta part of this patch for the rest of commit body.