Allow unique prefix for command line options, like any GNU program.

mysys/my_getopt.c

@@ -869,12 +869,6 @@ static int findopt(char *optpat, uint length,
       }
     }
   }
-  if (is_prefix && count == 1)
-    my_getopt_error_reporter(WARNING_LEVEL,
-                             "Using unique option prefix %.*s instead of %s "
-                             "is deprecated and will be removed in a future "
-                             "release. Please use the full name instead.",
-                             length, optpat, *ffname);
   DBUG_RETURN(count);
 }