sfc: fix unused-but-set-variable warning in efx_farch_filter_remove_safe

Thanks to some past refactor, 'spec' is not actually used in this
 function; the code using it moved to the callee efx_farch_filter_remove.
Remove the variable to fix a W=1 warning.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>