fm10k: remove extraneous NULL check on l2_accel

l2_accel was checked for NULL at the top of fm10k_dfwd_del_station, and
we return if it is not defined. Due to this, we already know it can't be
null here so a separate check is meaningless. Discovered via cppcheck.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>