target_configs.json: Remove unreachable nsapi entries
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.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed