[AArch64] Remove unneeded ExternalSymbolSDNode code for machine constraint "S". NFC

ExternalSymbolSDNode is implicitly generated libcalls but with an address taking
operation we cannot reference an ExternalSymbolSDNode.

Author: MaskRay

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -8160,10 +8160,6 @@ void AArch64TargetLowering::LowerAsmOperandForConstraint(
                    dyn_cast<BlockAddressSDNode>(Op)) {
       Result =
           DAG.getTargetBlockAddress(BA->getBlockAddress(), BA->getValueType(0));
-    } else if (const ExternalSymbolSDNode *ES =
-                   dyn_cast<ExternalSymbolSDNode>(Op)) {
-      Result =
-          DAG.getTargetExternalSymbol(ES->getSymbol(), ES->getValueType(0));
     } else
       return;
     break;