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/lets_split.h

@@ -1,5 +1,4 @@
-#ifndef LETS_SPLIT_H
-#define LETS_SPLIT_H
+#pragma once
 
 #include "quantum.h"
 
@@ -27,5 +26,3 @@
     )
 
 #define LAYOUT_kc_ortho_4x12 LAYOUT_kc
-
-#endif