Summary:
When `load_balancer_count_move_as_add` is set to true, we do the add and remove steps of
moving replicas from one tablet server to another in separate cluster balancer runs. This flag is
never set to false (even in tests), and somewhat complicates the implementation of the cluster
balancer. This diff deprecates the flag and simplifies the code it necessitated.

It also separates `SendReplicaChanges` which handles adding replicas, removing replicas, and leader moves into 3 separate functions.
Jira: DB-15602

Test Plan: existing tests

Reviewers: jhe

Reviewed By: jhe

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D42259