[async REST] Remove guards for async rest transport and async client generation once async REST support is GA

[ohmayr]

Once support for async REST transport goes from experimental -> GA, we need to remove the following conditions that guard the generation of the async rest transport and the async client:

from rest_asyncio.py.j2:

{% set rest_async_io_enabled = api.all_library_settings[api.naming.proto_package].python_settings.experimental_features.rest_async_io_enabled %}
{% if rest_async_io_enabled %}
..code
{% endif %}{# if rest_async_io_enabled #}

from async_client.py.j2:

{% set rest_async_io_enabled = api.all_library_settings[api.naming.proto_package].python_settings.experimental_features.rest_async_io_enabled %}
{% set is_grpc_transport = True if 'grpc' in opts.transport else False %}
{% if rest_async_io_enabled or is_grpc_transport %}
..code
{% endif %}{# if rest_async_io_enabled and is_grpc_transport #}

Making this change wil generate the relevant files i.e rest_asyncio.py and async_client.py unconditionally for GAPICs.

[vchudnov-g]

And more generally, we need to clean up any section of the code that has a TODO referencing this issue.