Remove ifq_drops from struct ifqueue. Now queue drops are accounted in
struct ifnet if_oqdrops.

Some netgraph modules used ifqueue w/o ifnet. Accounting of queue drops
is simply removed from them. There were no API to read this statistic.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Mechanically convert to if_inc_counter().
Mechanically convert to if_inc_counter().
Mechanically convert to if_inc_counter().
Mechanically convert to if_inc_counter().
Mechanically convert to if_inc_counter().