AIO_runtime_apihost should not be present in .env

`AIO_runtime_apihost` should be removed from the `.env` files generated by `aio app use` and `aio app init`.

Environment variables should be reserved for non-default values.

Note: The reason this variable cannot currently be removed is because `wskdebug` uses it to get openwhisk server runtime docker image data. We will need to find a way to get `wskdebug` this value without including it as part of `.env`.