i40e: i40e_fcoe.c: Remove unused function

Remove the function i40e_rx_is_fip() that is not used anywhere.

This was partially found by using a static code analysis program
called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>