Remove nrf51_dk from supported platform

Author: bulislaw

targets/targets.json

@@ -3416,7 +3416,7 @@
         "supported_form_factors": ["ARDUINO"],
         "inherits": ["MCU_NRF51_32K_UNIFIED"],
         "device_has": ["ANALOGIN", "I2C", "I2C_ASYNCH", "INTERRUPTIN", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SERIAL_FC", "SLEEP", "SPI", "SPI_ASYNCH", "SPISLAVE"],
-        "release_versions": ["2", "5"],
+        "release_versions": ["2"],
         "device_name": "nRF51822_xxAA"
     },
     "NRF51_DONGLE": {