[lld-macho][nfc] lld/Common's `demangle()` is redundant for Mach-O

The only thing that the Common implementation does is a check for
`config->demangle`, but {D135189} added that check to
`maybeDemangleSymbol`, so there's no need to go through `Common`...

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D135942