[X86] Remove broken support for autoupgrading llvm.x86.fma4.* intrinsics to llvm.x86.fma.*.

It currently fires an assert if you even try. Looking back, I don't think it ever worked because it only changed the name of the function object, but not the intrinsic ID stored in it. Given that, I think it can be removed since no one has noticed or complained in the past 4 years.

llvm-svn: 286031