Remove deprecated `led_set_user` (#23979)

docs/features/led_indicators.md

···
 The `host_keyboard_led_state()` may reflect an updated state before `led_update_user()` is called.
 :::
 
-Two deprecated functions that provide the LED state as `uint8_t`:
+Deprecated functions that provide the LED state as `uint8_t`:
 
-* `uint8_t led_set_user(uint8_t usb_led)`
 * `uint8_t host_keyboard_leds()`
 
 ## Configuration Options
···
 * Keymap: `bool led_update_user(led_t led_state)`
 
 Both receives LED state as a struct parameter. Returning `true` in `led_update_user()` will allow the keyboard level code in `led_update_kb()` to run as well. Returning `false` will override the keyboard level code, depending on how the keyboard level function is set up.
-
-::: tip
-This boolean return type of `led_update_user` allows for overriding keyboard LED controls, and is thus recommended over the void `led_set_user` function.
-:::
 
 ### Example of keyboard LED update implementation
 

keyboards/converter/usb_usb/custom_matrix.cpp

···
         if (kbd2.isReady()) kbd2.SetReport(0, 0, 2, 0, 1, &usb_led);
         if (kbd3.isReady()) kbd3.SetReport(0, 0, 2, 0, 1, &usb_led);
         if (kbd4.isReady()) kbd4.SetReport(0, 0, 2, 0, 1, &usb_led);
-        led_set_user(usb_led);
         led_update_kb((led_t){.raw = usb_led});
     }
 }

keyboards/sirius/unigo66/custom_matrix.cpp

···
         kbd2.SetReport(0, 0, 2, 0, 1, &usb_led);
         kbd3.SetReport(0, 0, 2, 0, 1, &usb_led);
         kbd4.SetReport(0, 0, 2, 0, 1, &usb_led);
-        led_set_user(usb_led);
         led_update_kb((led_t){.raw = usb_led});
     }
 

quantum/led.c

···
 #endif
 
 static uint32_t last_led_modification_time = 0;
-uint32_t        last_led_activity_time(void) {
+
+uint32_t last_led_activity_time(void) {
     return last_led_modification_time;
 }
+
 uint32_t last_led_activity_elapsed(void) {
     return timer_elapsed32(last_led_modification_time);
 }
-
-/** \brief Lock LED set callback - keymap/user level
- *
- * \deprecated Use led_update_user() instead.
- */
-__attribute__((weak)) void led_set_user(uint8_t usb_led) {}
 
 /** \brief Lock LED update callback - keymap/user level
  *
···
     handle_backlight_caps_lock((led_t)usb_led);
 #endif
 
-    led_set_user(usb_led);
     led_update_kb((led_t)usb_led);
 }
 

quantum/led.h

···
 
 void led_task(void);
 
-/* Deprecated callbacks */
-void led_set_user(uint8_t usb_led);
-
 /* Callbacks */
 bool led_update_user(led_t led_state);
 bool led_update_kb(led_t led_state);