Remove VMS remnants
Remove dec-mcs remnants

It was (apparently) the default character encoding file used for line
printing on VMS, which is unsupported.
Remove Amiga remnants

Notes regarding the removal of specific items:

  - Aztec C: only on the Amiga.
  - mch_check_win(): doesn't exist anymore.
  - Comment in ex_cmds.c: It seems the context for this comment was
    removed, but the comment was inadvertantly left alone.