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/ex_docmd.c

@@ -5141,8 +5141,7 @@ static void ex_quit(exarg_T *eap)
  */
 static void ex_cquit(exarg_T *eap)
 {
-  getout(1);    /* this does not always pass on the exit code to the Manx
-                   compiler. why? */
+  getout(1);
 }
 
 /*