sfc: Remove efx_channel::has_interrupt

efx_channel::has_interrupt is redundant with efx_channel::used_flags.

Remove efx_test_eventq() because it is now obviously unreachable.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>