ref(subscriptions): Remove the Arroyo subscriptions consumers

[lvthanh03]

Refs STREAM-1208

We have rolled out the subscriptions taskbroker raw mode to replace consumers in all regions, and the consumer code is good to be removed.

[linear-code]

STREAM-1208

[github-actions]

Backend Test Failures

Failures on 34087da in this run:

tests/snuba/incidents/test_tasks.py::HandleSnubaQueryUpdateTest::test_arroyo — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
src/sentry/consumers/__init__.py:503: in get_stream_processor
    consumer_definition = KAFKA_CONSUMERS[consumer_name]
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E   KeyError: 'metrics-subscription-results'

During handling of the above exception, another exception occurred:
tests/snuba/incidents/test_tasks.py:196: in test_arroyo
    consumer = get_stream_processor(
src/sentry/consumers/__init__.py:505: in get_stream_processor
    raise click.ClickException(
E   click.exceptions.ClickException: No consumer named metrics-subscription-results in sentry.consumers.KAFKA_CONSUMERS. Most likely there is another subcommand in 'sentry run' responsible for this consumer