Remove SOFT_SERIAL_PIN for non-split boards (#19774)

* Remove `SOFT_SERIAL_PIN` for non-split boards

* Delete more empty config.h

keyboards/splitkb/kyria/config.h

@@ -1,17 +0,0 @@
-/* Copyright 2022 splitkb.com <[email protected]>
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 2 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program.  If not, see <http://www.gnu.org/licenses/>.
- */
-
-#pragma once