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/nano_g2_ultra/nano-g2.h

@@ -35,9 +35,9 @@
 #define PIN_VBAT_READ            (0 + 2)
 #define REAL_VBAT_MV_PER_LSB     (VBAT_DIVIDER_COMP * VBAT_MV_PER_LSB)
 
-class NanoG2Ultra : public NRF52BoardOTA {
+class NanoG2Ultra : public NRF52Board {
 public:
-  NanoG2Ultra() : NRF52BoardOTA("NANO_G2_OTA") {}
+  NanoG2Ultra() : NRF52Board("NANO_G2_OTA") {}
   void begin();
   uint16_t getBattMilliVolts() override;