ethdev: remove detachable device flag

This flag is not necessary at the ether layer anymore.
Buses are able to advertise their hotplug support. The ether layer can
rely upon this capability instead of a special flag.

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>