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/rules.mk

@@ -1,11 +1,4 @@
-SRC += matrix.c \
-	   i2c.c \
-	   split_util.c \
-	   serial.c \
-	   ssd1306.c
-
 # MCU name
-#MCU = at90usb1287
 MCU = atmega32u4
 
 # Processor frequency.
@@ -42,7 +35,7 @@ F_USB = $(F_CPU)
 
 # Bootloader
 #     This definition is optional, and if your keyboard supports multiple bootloaders of
-#     different sizes, comment this out, and the correct address will be loaded 
+#     different sizes, comment this out, and the correct address will be loaded
 #     automatically (+60). See bootloader.mk for all options.
 BOOTLOADER = caterina
 
@@ -64,13 +57,12 @@ MIDI_ENABLE = no            # MIDI controls
 AUDIO_ENABLE = no           # Audio output on port C6
 UNICODE_ENABLE = no         # Unicode
 BLUETOOTH_ENABLE = no       # Enable Bluetooth with the Adafruit EZ-Key HID
-RGBLIGHT_ENABLE = no       # Enable WS2812 RGB underlight. 
-SUBPROJECT_rev1 = yes
-USE_I2C = yes
+RGBLIGHT_ENABLE = no       # Enable WS2812 RGB underlight.
+
 # Do not enable SLEEP_LED_ENABLE. it uses the same timer as BACKLIGHT_ENABLE
 SLEEP_LED_ENABLE = no    # Breathing sleep LED during USB suspend
 
-CUSTOM_MATRIX = yes
+SPLIT_KEYBOARD = yes
 
 LAYOUTS = ortho_4x12