Delete MockDeploymentService records

What does this MR do?

Delete MockDeploymentService records (when sufficient numbers have run the delete migration, will delete the MockDeploymentService model in #293914 (closed)). Migration will only run in development environment.

Migration output:

$ be rails db:migrate:redo
== 20201215084652 DeleteMockDeploymentServiceRecords: reverting ===============
== 20201215084652 DeleteMockDeploymentServiceRecords: reverted (0.0000s) ======

== 20201215084652 DeleteMockDeploymentServiceRecords: migrating ===============
-- execute("DELETE FROM services WHERE type = 'MockDeploymentService'")
   -> 0.0026s
== 20201215084652 DeleteMockDeploymentServiceRecords: migrated (0.0026s) ======

And remove un-used code. This class became unusable since DeploymentService was deleted in 659c6e6b.