Keyboard: Refactor lets split to use split common code (#4536)

* Refactor lets split to use split common code

* Refactor lets split to use split common code

* Build fixes for OLED_sample keymap

* Build fixes for OLED_sample keymap - attempt to reduce firmware size

keyboards/lets_split/config.h

@@ -16,9 +16,6 @@ You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#ifndef CONFIG_H
-#define CONFIG_H
+#pragma once
 
 #include "config_common.h"
-
-#endif