[lld][nfc] Remove lld::demangle() (partial revert of D116279)

{D116279}, in addition to adding support for other demanglers, also
factored out some of the demangling logic. However, I don't think the
abstraction really carries its weight -- after {D135942}, only the ELF
and WASM backends call it with anything other than a non-constant
`shouldDemangle` argument. The COFF and Mach-O backends were already
doing the should-demangle check before calling `demangle()`.

Reviewed By: MaskRay, #lld-macho

Differential Revision: https://reviews.llvm.org/D135943