Remove SOFT_SERIAL_PIN for non-split boards (#19774)

* Remove `SOFT_SERIAL_PIN` for non-split boards

* Delete more empty config.h

keyboards/lfkeyboards/lfk78/config.h

@@ -21,11 +21,6 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 /* COL2ROW, ROW2COL*/
 #define DIODE_DIRECTION COL2ROW
 
-/*
- * Split Keyboard specific options, make sure you have 'SPLIT_KEYBOARD = yes' in your rules.mk, and define SOFT_SERIAL_PIN.
- */
-#define SOFT_SERIAL_PIN D0  // or D1, D2, D3, E6
-
 #define BACKLIGHT_LEVELS 8
 #define BACKLIGHT_PWM_MAP { 8, 16, 40, 55, 70, 128, 200, 255 }