tcp: Remove redundant code entering quickack mode

tcp_enter_quickack_mode() already calls tcp_incr_quickack() and sets
icsk->icsk_ack.ato  to TCP_ATO_MIN. This patch removes the duplication.

Signed-off-by: Vijay Subramanian <subramanian.vijay@gmail.com>
Reviewed-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>