swift234

SILGen: Remove PolymorphicFunctionType usage from materializeForSet emission, NFC

We always passed nullptr to getMaterializeForSetCallbackType(), so
I'm assuming it is safe to just delete this bit of dead code.

Now the only remaining usages of PolymorphicFunctionType in SIL are
assertions ensuring that PolymorphicFunctionType doesn't end up there.

There are still a number of usages of AllArchetypes though, via
SILFunction::ContextGenericParams. Once @rjmccall finishes the
heavy lifting on the IRGen side, it should be pretty easy to rip
these out from SIL, too.