Remove Clusters::Application database tables

Once Clusters::Applications models removal (#388696 - closed) is finished we need to follow procedures in

https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-tables

and

https://docs.gitlab.com/ee/development/database/database_dictionary.html#dropping-tables

for all the tables related to Clusters::Applications models removal (#388696 - closed)