Remove deprecated server pool references from OM 4.2 docs

Description

Today I was trying to point a customer to how to provision hosts for automation, and noticed that server pools were still mentioned in this section: https://docs.opsmanager.mongodb.com/current/tutorial/nav/add-servers/

Server pools are deprecated in 4.2 and are no longer visible in the dashboard at all, unless you were using the feature in 4.0 and then upgraded. New users will be confused by this, so the pages should be removed entirely, or a big warning posted on top. I would just remove it since I do not believe it is possible even for existing users to continue provisioning new servers for the pools.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)