Remove unused RX boosted gain mode functions and related preprocessor directives across multiple variants

variants/xiao_nrf52/target.h

@@ -25,7 +25,3 @@ void radio_set_params(float freq, float bw, uint8_t sf, uint8_t cr);
 void radio_set_tx_power(int8_t dbm);
 mesh::LocalIdentity radio_new_identity();
 
-#if defined(USE_SX1262) || defined(USE_SX1268)
-bool radio_get_rx_boosted_gain_mode();
-void radio_set_rx_boosted_gain_mode(bool rxbgm);
-#endif