[lib] Remove assertMessageStoreMessagesAreEqual(...) from message-reducer

Summary:
We no longer need to assert that the "old approach" and the "ops approach" are equal... because we're removing the "old approach" altogether.

This diff

• removes the last usage of assertMessageStoreMessagesAreEqual from mergeNewMessages
• removes assertMessageStoreMessagesAreEqual
• removes imports that assertMessageStoreMessagesAreEqual depended on
• removes actionType argument from mergeNewMessages(..._) since it was only used by assertMessageStoreMessagesAreEqual from within mergeNewMessages

Depends on D4332

Test Plan: This should be a straightforward refactor given we've had assertMessageStoreMessagesAreEqual in place for some time now.

Reviewers: def-au1t, palys-swm, yayabosh

Reviewed By: palys-swm, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4333