Remove support for Reach provider

This provider was never active and support was never completed, so
there's little value in keeping all this potentially confusing code.

app/celery/process_sms_client_response_tasks.py

@@ -8,7 +8,6 @@ from app import notify_celery, statsd_client
 from app.clients import ClientException
 from app.clients.sms.firetext import get_firetext_responses
 from app.clients.sms.mmg import get_mmg_responses
-from app.clients.sms.reach import get_reach_responses
 from app.dao import notifications_dao
 from app.dao.templates_dao import dao_get_template_by_id
 from app.models import NOTIFICATION_PENDING
@@ -19,7 +18,6 @@ from app.notifications.notifications_ses_callback import (
 sms_response_mapper = {
     'MMG': get_mmg_responses,
     'Firetext': get_firetext_responses,
-    'Reach': get_reach_responses
 }