Remove SERIAL_LINK feature (#14727)

* Remove SERIAL_LINK

* more stale paths in doxygen-todo

* Fix

* More refs

* Update testing docs

* Update doxygen-todo

Co-authored-by: Ryan <fauxpark@gmail.com>

Co-authored-by: Ryan <fauxpark@gmail.com>

quantum/keyboard.c

@@ -61,9 +61,6 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #ifdef STENO_ENABLE
 #    include "process_steno.h"
 #endif
-#ifdef SERIAL_LINK_ENABLE
-#    include "serial_link/system/serial_link.h"
-#endif
 #ifdef POINTING_DEVICE_ENABLE
 #    include "pointing_device.h"
 #endif
@@ -511,10 +508,6 @@ MATRIX_LOOP_END:
     adb_mouse_task();
 #endif
 
-#ifdef SERIAL_LINK_ENABLE
-    serial_link_update();
-#endif
-
 #ifdef POINTING_DEVICE_ENABLE
     pointing_device_task();
 #endif