fix: V4 also has plain LED on GPIO35, not WS2812 on GPIO38

Per the official V4 schematic (HTIT-WB32LAF_V4.3.pdf), the V4 has
an orange LED on GPIO35 with a series resistor — same setup as V3.
GPIO38 is GNSS_RX on V4, not an LED. Neither V3 nor V4 has a WS2812.

- V4: switched from WS2812/GPIO38 to SimpleLed/GPIO35
- Removed ws2812.rs driver (no current board uses addressable LEDs)
- All ESP boards now use the same SimpleLed driver on GPIO35

src/driver/mod.rs

@@ -1,8 +1,5 @@
 #[cfg(feature = "wio_tracker_l1")]
 pub mod sh1106;
 
-#[cfg(any(feature = "heltec_v3", feature = "heltec_v3_uart"))]
+#[cfg(any(feature = "heltec_v3", feature = "heltec_v3_uart", feature = "heltec_v4"))]
 pub mod simple_led;
-
-#[cfg(feature = "heltec_v4")]
-pub mod ws2812;