Fix some warnings

Remove an unused variable, add missing FALLTHRUs, and use the same
FALLTHRU wording where it wasn't recognized by gcc

src/steed.c

@@ -485,6 +485,7 @@ int reason; /* Player was thrown off etc. */
     switch (reason) {
     case DISMOUNT_THROWN:
         verb = "are thrown";
+        /*FALLTHRU*/
     case DISMOUNT_FELL:
         You("%s off of %s!", verb, mon_nam(mtmp));
         if (!have_spot)