[lld-macho][nfc] Misc code cleanup

* Move `static_asserts` into cpp instead of header file. I noticed they
  had been separated from the main class definition in the header, so I
  set about to clean that up, then figured it made more sense as part of
  the cpp file so as not to incur unnecessary compile-time overhead.

* Remove unnecessary `virtual`s

* Remove unnecessary comment / reword another comment