[clang] [AIX][clang][driver] fix no-pthread option (PR #69363)

[Sam Clegg via cfe-commits]

sbc100 wrote:

FWIW, I won't think this flag was every supposed to have any effect on any platform.    It was added back in https://github.com/llvm/llvm-project/commit/425b1a5106def460811aaf6a472750f2524b7134 so that the driver would at least accept and ignore this flag to support mingw-w64 + winthreads. 

However, I don't think was ever really supported and I just make the opposite change to the Wasm clang driver to match the behavior (https://github.com/llvm/llvm-project/pull/216378).  There are now zero references to `OPT_no_pthread` in clang.  I would recommend against adding one back and instead close this PR.

https://github.com/llvm/llvm-project/pull/69363