Remove SvIsCOW checks from mg.c:mg_localize

Remove SvIsCOW checks from mg.c:mg_localize

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 discrepancies with scalar-tied locked hash keys,
which differ in readonliness when localised depending on whether the previous value used copy-on-write.

Whether such scalars should be read-only after localisation is open
to debate, but it should not differ based on the means of storing the
previous value.