Remove SOFT_SERIAL_PIN for non-split boards (#19774)

* Remove `SOFT_SERIAL_PIN` for non-split boards

* Delete more empty config.h

keyboards/lets_split/config.h

@@ -1,20 +0,0 @@
-/*
-Copyright 2012 Jun Wako <[email protected]>
-Copyright 2015 Jack Humbert
-
-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
-