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.

src/nvim/testdir/test17.in

@@ -16,11 +16,7 @@ STARTTEST
 :let $CDIR = "."
 /CDIR
 :else
-:if has("amiga")
-:let $TDIR = "/testdir"
-:else
 :let $TDIR = "."
-:endif
 /TDIR
 :endif
 :" Dummy writing for making that sure gf doesn't fail even if the current