Disable BLE_PIN_CODE for E290 companion USB environment

Remove the hardcoded BLE_PIN_CODE build flag from the Heltec_E290_companion_ble
environment. The presence of BLE_PIN_CODE prevents the unit from operating as a
USB companion. Commenting it out allows dynamic PIN generation and enables USB
companion functionality.

variants/heltec_e290/platformio.ini

@@ -66,7 +66,7 @@ build_flags =
   -D MAX_GROUP_CHANNELS=40
   -D DISPLAY_CLASS=E290Display
   -D AUTO_OFF_MILLIS=0
-  -D BLE_PIN_CODE=123456   ; dynamic, random PIN
+  #-D BLE_PIN_CODE=123456   ; dynamic, random PIN
   -D BLE_DEBUG_LOGGING=1
   -D OFFLINE_QUEUE_SIZE=256
 build_src_filter = ${Heltec_E290_base.build_src_filter}