Remove Project#cleanup_chat_names method and index

What does this MR do?

I've just seen this method in the codebase that I think we can remove now.

• Remove Project#cleanup_chat_names method.
• Remove index from chat_names and service_id.

Migration script output

$ rails db:migrate
== 20200727082309 RemoveIndexChatNameServiceId: migrating =====================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:chat_names, :service_id, {:algorithm=>:concurrently})
  1 Remove index from chat_names and service_id
   -> 0.0032s
-- remove_index(:chat_names, {:algorithm=>:concurrently, :column=>:service_id})
   -> 0.0071s
== 20200727082309 RemoveIndexChatNameServiceId: migrated (0.0107s) ============

$ rails db:rollback
== 20200727082309 RemoveIndexChatNameServiceId: reverting =====================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:chat_names, :service_id, {:algorithm=>:concurrently})
   -> 0.0032s
-- add_index(:chat_names, :service_id, {:algorithm=>:concurrently})
   -> 0.0063s
== 20200727082309 RemoveIndexChatNameServiceId: reverted (0.0099s) ============

Related to #204787 (closed)