[PM] Remove the (deeply misguided) 'unregister' functionality from the

pass registry.

This style of registry is somewhat questionable, but it being
non-monotonic is crazy. No one is (or should be) unloading DSOs with
passes and unregistering them here. I've checked with a few folks and
I don't know of anyone using this functionality or any important use
case where it is necessary.

llvm-svn: 219096