sfc: Remove bogus call to efx_release_tx_buffers()

efx_unregister_netdev() should not call efx_release_tx_buffers()
directly, as it is already done when closing the device:
efx_net_stop() -> efx_stop_all() -> efx_stop_datapath() ->
efx_fini_tx_queue() -> efx_release_tx_buffers().

(This was presumably a workaround for a race between efx_stop_all()
and the data path that has since been properly fixed.)

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