[CodeGen] Remove an unused #include (NFC) (llvm#164569)

We've switched to llvm::identity_cxx20 for SparseMultiSet, so we don't
need llvm::identity in this file.

Author: kazutakahirata

llvm/include/llvm/CodeGen/ScheduleDAGInstrs.h

@@ -18,7 +18,6 @@
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/SparseMultiSet.h"
-#include "llvm/ADT/identity.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/CodeGen/LiveRegUnits.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"