[NFC] Fix Sanitizer breakage introduced in llvm#128166 (llvm#128990)

Remove accidental leftover unused variable.

Author: AlexVlx

clang/lib/CodeGen/CGExprAgg.cpp

@@ -303,7 +303,6 @@ void AggExprEmitter::withReturnValueSlot(
   llvm::IntrinsicInst *LifetimeStartInst = nullptr;
   if (!UseTemp) {
     RetAddr = Dest.getAddress();
-    RawAddress RetAllocaAddr = RawAddress::invalid();
   } else {
     RetAddr = CGF.CreateMemTempWithoutCast(RetTy, "tmp");
     llvm::TypeSize Size =