Revert "[MC] Explicitly mark MCSymbol for MO_ExternalSymbol" (#133291)

Reverts llvm/llvm-project#108880 .

The patch has no regression test, no description of why the fix is
necessary, and the code is modifying MC datastructures in a way that's
forbidden in the AsmPrinter.

Fixes #132055.

(cherry picked from commit cd6e959)

Author: efriedma-quic

llvm/lib/Target/X86/X86MCInstLower.cpp

@@ -348,12 +348,8 @@ MCOperand X86MCInstLower::LowerMachineOperand(const MachineInstr *MI,
     return MCOperand::createImm(MO.getImm());
   case MachineOperand::MO_MachineBasicBlock:
   case MachineOperand::MO_GlobalAddress:
+  case MachineOperand::MO_ExternalSymbol:
     return LowerSymbolOperand(MO, GetSymbolFromOperand(MO));
-  case MachineOperand::MO_ExternalSymbol: {
-    MCSymbol *Sym = GetSymbolFromOperand(MO);
-    Sym->setExternal(true);
-    return LowerSymbolOperand(MO, Sym);
-  }
   case MachineOperand::MO_MCSymbol:
     return LowerSymbolOperand(MO, MO.getMCSymbol());
   case MachineOperand::MO_JumpTableIndex: