Next set of split_common changes (#4974)

* Update split_common to use standard i2c drivers

* Eliminate RGB_DIRTY/BACKLIT_DIRTY

* Fix avr i2c_master error handling

* Fix i2c_slave addressing

* Remove unneeded timeout on i2c_stop()

* Fix RGB I2C transfers

* Remove incorrect comment

quantum/quantum.h

@@ -44,10 +44,6 @@
     #endif
 #endif
 
-#ifdef SPLIT_KEYBOARD
-    #include "split_flags.h"
-#endif
-
 #ifdef RGB_MATRIX_ENABLE
     #include "rgb_matrix.h"
 #endif