ice: Remove unused ICE_FLOW_SEG_HDRS_L2_MASK

Remove the unused define ICE_FLOW_SEG_HDRS_L2_MASK.

Reported-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Acked-by: Paul Menzel <pmenzel@molgen.mpg.de>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>