Remove experimental GraphDriver plugin type

Signed-off-by: Linus Heckemann <anonymouse2048@gmail.com>

docs/extend/plugin_api.md

@@ -129,7 +129,6 @@ After activation, the plugin will then be sent events from this subsystem.
 
 Possible values are:
  - [`authz`](authorization.md)
- - `GraphDriver`
  - [`NetworkDriver`](plugins_network.md)
  - [`VolumeDriver`](plugins_volume.md)