Remove IIC_DEFAULT from X86Schedule.td

All the instructions tagged with IIC_DEFAULT had nothing in common, and
we already have a NoItineraries class to represent untagged
instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177937 91177308-0d34-0410-b5e6-96231b3b80d8