sfc: move some ARFS code out of headers

efx_filter_rfs_expire() is a work-function, so it being inline makes no
 sense.  It's only ever used in efx_channels.c, so move it there.
While we're at it, clean out some related unused cruft.

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