Moving IPVLAN driver out of experimental

IPVLAN driver had been retained in experimental for multiple releases
with the requirement to have a proper L3 control-plane (such as BGP) to
go along with it which will make this driver much more useful. But
based on the community feedback,
moby/moby#21735, am proposing to move this
driver out of experimental.

Signed-off-by: Madhu Venugopal <madhu@docker.com>

drivers_experimental_linux.go

@@ -1,9 +1,5 @@
 package libnetwork
 
-import "github.com/docker/libnetwork/drivers/ipvlan"
-
 func additionalDrivers() []initializer {
-	return []initializer{
-		{ipvlan.Init, "ipvlan"},
-	}
+	return []initializer{}
 }