I've made some changes in case someone decides to add this code to Marlin, but not enable it:

case 28: //G28 Home all Axis one at a time
#ifdef ENABLE_AUTO_BED_LEVELING
#ifndef SAVE_G29_CORRECTION_MATRIX
plan_bed_level_matrix.set_to_identity(); //Reset the plane ("erase" all leveling data)
#endif
#endif //ENABLE_AUTO_BED_LEVELING

#if Z_MIN_PIN == -1
#error "You must have a Z_MIN endstop in order to enable Auto Bed Leveling feature!!! Z_MIN_PIN must point to a valid hardware pin."
#endif
#ifdef SAVE_G29_CORRECTION_MATRIX
plan_bed_level_matrix.set_to_identity(); //Reset the plane ("erase" all leveling data)
#endif

This way G28's original function of erasing the matrix is retained while the code is disabled, and then switched to G29 when it is enabled.