Either all skills should have 1/5 efficiency on sundered monsters or none

[Sackett-11374]

@Numinor-2629

Make cold mastery efficiency 1/5 too, or revert the change, otherwise cold sorcs are big-big winners of this patch and fire once again in the dumpster.

Only if Cold mastery is able to break Immunities on its own like Conviction and Lower resist.