ionic: no rx flush in deinit

Kmemleak pointed out to us that ionic_rx_flush() is sending
skbs into napi_gro_XXX with a disabled napi context, and these
end up getting lost and leaked.  We can safely remove the flush.

Fixes: 0f3154e6 ("ionic: Add Tx and Rx handling")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>