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

[weiwei chen via llvm-commits]

weiweichen wrote:

Ah, here is more info on the error from ORC JIT. Without this change, if we try to execute generated object file through ORC JIT, ti complains about a duplicated external symbol error

```
external/+llvm_configure+llvm-project/llvm/include/llvm/ExecutionEngine/JITLink/JITLink.h:1295: Symbol &llvm::jitlink::LinkGraph::addExternalSymbol(orc::SymbolStringPtr, orc::ExecutorAddrDiff, bool): Assertion `!ExternalSymbols.contains(*Name) && "Duplicate external symbol"' failed.
```

https://github.com/llvm/llvm-project/pull/133291