sv.c: Don’t fiddle with AMAGIC in sv_bless

sv.c: Don’t fiddle with AMAGIC in sv_bless

Since overloading itself now checks whether caches are up to date, and
since changes to the stash (@ISA, methods) turn the flag on and over-
loading itself turns the flag off when it can, sv_bless no longer
needs to deal with it at all.