Dropping system.views should be allowed

There is a left-over check that prohibits dropping system.views while in 3.4 compatibility mode, as at one point the presence of a system.views collection was used to make sure initial sync on 3.2 would fail in 3.4 compatibility mode. However, now we use V2 indexes for that, so this is no longer needed.

Additionally, the design of views requires them to be droppable, as replication may need to drop a system.views collection during rollback. We should also add a test for this.