[llvm] [BOLT][NFC] Clean up the outdated option --write-dwp in doc (PR #166150)

[Paschalis Mpeis via llvm-commits]

https://github.com/paschalis-mpeis approved this pull request.

Looks good. Maybe worth removing the usage of the *unsupported* test in `bolt/test/X86/dwarf4-ftypes-dwp-input-dwp-output.test` too?

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