[llvm] [BOLT][NFC] Clean up the outdated option --write-dwp in doc (PR #166150)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

Thanks Jinjie. I was referring only to the flag on [line 11](https://github.com/llvm/llvm-project/pull/166150/commits/162bf742c79334fd1ef6ecf6df2b4ca3e368c35e#diff-2de1c2abd034ac2f6c7d71ff967280bda43efa94e9b5727d6133b3552056fc8aL11), not to the whole (unsupported) test.
I'll defer to others if they're fine with removing it (cc: @ayermolo, @grigorypas).

https://github.com/llvm/llvm-project/pull/166150