igb: Remove unnecessary flag setting in igb_set_flag_queue_pairs()

If VFs are enabled (max_vfs >= 1), both max_rss_queues and
adapter->rss_queues are set to 2 in the case of e1000_82576.
In this case, IGB_FLAG_QUEUE_PAIRS is always set in the default block as a
result of fall-through, thus setting it in the e1000_82576 block is not
necessary.

Signed-off-by: Shota Suzuki <suzuki_shota_t3@lab.ntt.co.jp>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>