Remove an unnecessary header from this file. I don't think this header

was really intended, and it may have been required prior to some of the
recent refactors. Including it however causes LLVMX86Desc to need
symbols from LLVMX86CodeGen, forming a dependency cycle. This was masked
in almost all builds: Clang, and GCC w/ optimizations didn't actually
emit the symbols!

llvm-svn: 135242