[X86] Remove useless target specific combine on TRUNCATE dag nodes.

Before revision 171146, function 'PerformTruncateCombine' used to perform
a premature lowering of TRUNCATE dag nodes.
Revision 171146 then moved all the logic implemented by PerformTruncateCombine
to a custom lowering hook. However, that revision forgot to delete
function PerformTruncateCombine from the code.

This patch removes function 'PerformTruncateCombine' since it has no effect
on the SelectionDAG. No functional change intended.

llvm-svn: 237122