Revert "Make action_for_key a weak symbol"

This reverts commit c17b8a599e.

The commit made compile error and its intention is unclear.

tmk_core/common/action.h

@@ -51,7 +51,6 @@ void action_exec(keyevent_t event);
 
 /* action for key */
 action_t action_for_key(uint8_t layer, keypos_t key);
-action_t action_for_key_default(uint8_t layer, keypos_t key);
 
 /* macro */
 const macro_t *action_get_macro(keyrecord_t *record, uint8_t id, uint8_t opt);

tmk_core/common/keymap.c

@@ -27,13 +27,8 @@ static action_t keycode_to_action(uint8_t keycode);
 
 
 /* converts key to action */
-__attribute__((__weak__))
 action_t action_for_key(uint8_t layer, keypos_t key)
 {
-	return action_for_key_default(layer, key);
-}
-
-action_t action_for_key_default(uint8_t layer, keypos_t key)
     uint8_t keycode = keymap_key_to_keycode(layer, key);
     switch (keycode) {
         case KC_FN0 ... KC_FN31: