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/thinknode_m1/ThinkNodeM1Board.h

@@ -13,9 +13,9 @@
 #define PIN_VBAT_READ     (4)
 #define REAL_VBAT_MV_PER_LSB (VBAT_DIVIDER_COMP * VBAT_MV_PER_LSB)
 
-class ThinkNodeM1Board : public NRF52BoardOTA {
+class ThinkNodeM1Board : public NRF52Board {
 public:
-  ThinkNodeM1Board() : NRF52BoardOTA("THINKNODE_M1_OTA") {}
+  ThinkNodeM1Board() : NRF52Board("THINKNODE_M1_OTA") {}
   void begin();
   uint16_t getBattMilliVolts() override;