Remove SvIsCOW checks from mg.c:S_save_magic

Remove SvIsCOW checks from mg.c:S_save_magic

It no longer needs to worry about SvIsCOW.  This logic is left over
from when READONLY+FAKE was used for COWs.

Since it is possible for COWs to be read-only now, this logic is actu-
ally faulty, as it doesn’t temporarily stop read-only COWs from being
read-only, as it does for other read-only values.

This actually causes bugs with scalar-tied locked hash keys, which
croak on FETCH.