Remove ERROR_FILTER_TOO_OLD error message

This error message is no longer used.

lang/strings_english.txt

@@ -1659,7 +1659,6 @@ $MANTIS_ERROR[ERROR_GPC_ARRAY_UNEXPECTED] = 'A string was expected but an array
 $MANTIS_ERROR[ERROR_GPC_NOT_NUMBER] = 'A number was expected for %1$s.';
 $MANTIS_ERROR[ERROR_BUG_NOT_FOUND] = 'Issue %1$d not found.';
 $MANTIS_ERROR[ERROR_FILTER_NOT_FOUND] = 'Filter %1$s not found.';
-$MANTIS_ERROR[ERROR_FILTER_TOO_OLD] = 'The filter you are trying to use is too old to be upgraded. Please re-create it.';
 $MANTIS_ERROR[ERROR_EMAIL_INVALID] = 'Invalid e-mail address.';
 $MANTIS_ERROR[ERROR_EMAIL_DISPOSABLE] = 'It is not allowed to use disposable e-mail addresses.';
 $MANTIS_ERROR[ERROR_USER_PROFILE_NOT_FOUND] = 'Profile not found.';