osd: cache pool: delete dead code in ReplicatedPG::agent_choose_mode

Signed-off-by: Xinze Chi <xmdxcxz@gmail.com>
Merge pull request #2860 from XinzeChi/master

osd: cache pool: delete dead code in ReplicatedPG::agent_choose_mode

Reviewed-by: Loic Dachary <ldachary@redhat.com>