[AArch64] Remove unused DenseMap variable. NFC

Author: topperc

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -7923,7 +7923,6 @@ SDValue AArch64TargetLowering::LowerFormalArguments(
 
   // Assign locations to all of the incoming arguments.
   SmallVector<CCValAssign, 16> ArgLocs;
-  DenseMap<unsigned, SDValue> CopiedRegs;
   CCState CCInfo(CallConv, isVarArg, MF, ArgLocs, *DAG.getContext());
 
   // At this point, Ins[].VT may already be promoted to i32. To correctly