MINOR: remove unnecessary judgment in AdminUtils::assignReplicasToBrokersRackAware

[JoeCqupt]

remove unnecessary judgment in method AdminUtils::assignReplicasToBrokersRackAware
because replicationFactor <= numBrokers , so the judgment is not necessary

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[JoeCqupt]

call for review @ijuma @guozhangwang @hachikuji @mjsax @ableegoldman

[guozhangwang]

Overall looks good to me. Just one qq: do we always guarantee replicationFactor <= numBrokers? I know we check and forbid it when creating a new topic, but after a topic is created, if we shutdown brokers and reassign replicas, would we stop with less replicas and warn users?

cc @ijuma for another look.

[JoeCqupt]

@ijuma