Summary:
The "use existing replica" code removed in this diff was added in https://phorge.dev.yugabyte.com/D6854 to fix a bug where the LB was not counting ongoing bootstraps as ongoing when a config change happened. This is indirectly a result of using RaftGroupStatePB instead of consensus state (which @jhe fixed in https://phorge.dev.yugabyte.com/D37189).
Jira: DB-13992

Test Plan: existing tests

Reviewers: zdrudi, jhe

Reviewed By: zdrudi

Subscribers: ybase, jhe

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D39681