Remove NRF52BoardOTA class and integrate it into NRF52Board

As all NRF52 boards now have OTA support, let's remove the subclass
and integrate it into the base class.

Signed-off-by: Frieder Schrempf <frieder@fris.de>

variants/heltec_mesh_solar/MeshSolarBoard.h

@@ -20,9 +20,9 @@
 #define SX126X_DIO2_AS_RF_SWITCH  true
 #define SX126X_DIO3_TCXO_VOLTAGE   1.8
 
-class MeshSolarBoard : public NRF52BoardDCDC, public NRF52BoardOTA {
+class MeshSolarBoard : public NRF52BoardDCDC {
 public:
-  MeshSolarBoard() : NRF52BoardOTA("MESH_SOLAR_OTA") {}
+  MeshSolarBoard() : NRF52Board("MESH_SOLAR_OTA") {}
   void begin();
 
   uint16_t getBattMilliVolts() override {