[docs] Remove outdated documentation for the legacy Atom-based LLD

The outdated documentation diverges a lot from the current state of
COFF/Mach-O/ELF/wasm ports and may just confuse users. It is better rewriting
some if useful.

Tested with `ninja docs-lld-html`

Reviewed By: #lld-macho, lhames, Jez Ng

Differential Revision: https://reviews.llvm.org/D113432