Disable Nordic SoftDevice BLE stack from targets (affects NRF51* targets)

The Nordic SoftDevice BLE stack used by NRF51* targets only
supports legacy BLE APIs which we will removed completely.

Note: NRF52* targets which use Cordio BLE stack are unaffected.

Author: LDong-Arm

targets/targets.json

@@ -10959,9 +10959,7 @@
             "MCU_NRF51",
             "MCU_NRF51822_UNIFIED",
             "NRF5x",
-            "NRF51",
-            "SDK_11",
-            "NORDIC_SOFTDEVICE"
+            "NRF51"
         ],
         "OUTPUT_EXT": "hex",
         "is_disk_virtual": true,
@@ -10991,9 +10989,6 @@
             ]
         },
         "program_cycle_s": 6,
-        "features": [
-            "BLE"
-        ],
         "config": {
             "lf_clock_src": {
                 "value": "NRF_LF_SRC_XTAL",