target_configs.json: Remove unreachable nsapi entries

In `tools/test_configs/`, target-specific test configurations are
defined in `target_configs.json` and parsed by `__init__.py`. The
latter only makes use of `default_test_configuration` (default test
configuration to use) and `test_configurations` (more configurations
selectable via `mbed test --test-config <config>`. Anything else
is ignored, including nsapi, so this commit cleans up dead entries.

Author: LDong-Arm

tools/test_configs/target_configs.json

@@ -1,21 +1,17 @@
 {
     "K64F": {
-        "nsapi.socket-stats-enable": true,
         "default_test_configuration": "HEAPBLOCKDEVICE_AND_ETHERNET",
         "test_configurations": ["HEAPBLOCKDEVICE_AND_ETHERNET", "NANOSTACK_MAC_TESTER", "ESP8266_WIFI", "ETHERNET"]
     },
     "NUCLEO_F429ZI": {
-        "nsapi.socket-stats-enable": true,
         "default_test_configuration": "HEAPBLOCKDEVICE_AND_ETHERNET",
         "test_configurations": ["HEAPBLOCKDEVICE_AND_ETHERNET", "NANOSTACK_MAC_TESTER"]
     },
     "DISCO_L475VG_IOT01A": {
-        "nsapi.socket-stats-enable": true,
         "default_test_configuration": "NONE",
         "test_configurations": ["ISM43362_WIFI"]
     },
     "DISCO_F413ZH": {
-        "nsapi.socket-stats-enable": true,
         "default_test_configuration": "NONE",
         "test_configurations": ["ISM43362_WIFI"]
     },