sfc: Remove efx_process_channel_now()

efx_process_channel_now() is unneeded since self-tests can rely on
normal NAPI polling.  Remove it and all calls to it.

efx_channel::work_pending and efx_channel_processed() are also
unneeded (the latter being the same as efx_nic_eventq_read_ack()).

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>