Keyboard: Refactor fourier to use split common code (#4582)

* Refactor fourier to use split common code

* Refactor fourier to use split common code - fix handedness when not using EE_HANDS or MASTER_LEFT

* add SOFT_SERIAL_PIN config

keyboards/fourier/keymaps/default/keymap.c

@@ -1,7 +1,5 @@
 #include QMK_KEYBOARD_H
 
-extern keymap_config_t keymap_config;
-
 // Each layer gets a name for readability, which is then used in the keymap matrix below.
 // The underscores don't mean anything - you can have a layer called STUFF or any other name.
 // Layer names don't all need to be of the same length, obviously, and you can also skip them
@@ -14,8 +12,6 @@ enum custom_keycodes {
   QWERTY = SAFE_RANGE,
 };
 
-#define _______ KC_TRNS
-#define XXXXXXX KC_NO
 #define KC_FN1 MO(_FN1)
 #define KC_FN2 MO(_FN2)
 #define KC_SPFN1 LT(_FN1, KC_SPACE)