Deduplicate reboot() for NRF52 boards

The reboot() method is the same for all NRF52 boards. Use a shared
implementation.

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

variants/lilygo_techo/TechoBoard.h

@@ -49,8 +49,4 @@ public:
     #endif
     sd_power_system_off();
   }
-
-  void reboot() override {
-    NVIC_SystemReset();
-  }
 };