Remove legacy USE_SERIAL define (#18292)

* Remove legacy USE_SERIAL define

* tidy up missed comments

keyboards/rgbkb/zen/rev1/keymaps/starcalleramethyst/config.h

@@ -1,5 +1,5 @@
 /*
-Copyright 2019 starcalleramethyst 
+Copyright 2019 starcalleramethyst
 
 This program is free software: you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
@@ -17,12 +17,6 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #pragma once
 
-
-/* Use I2C or Serial, not both */
-
-#define USE_SERIAL
-// #define USE_I2C
-
 /* Select hand configuration */
 
 #define MASTER_LEFT