igbvf: remove unused spinlock

tx_queue_lock and stats_lock are declared and initialized, but never
used. Remove them.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>