mana: remove redundant doorbell in mana_poll_rx_cq()

With the last commit to refill the rx mbuf in batch, the doorbell
in mana_poll_rx_cq() becomes redundant. Remove it to save a few
microseconds spent in mmio call.

Reported by: NetApp
Reviewed by: Tallamraju, Sai
Tested by: whu
Fixes: 9b8701b8 ("mana: refill the rx mbuf in batch")
MFC after: 3 days
Sponsored by: Microsoft