lxd/instances: Don't bypass instance limit check (from Incus) (#14021)

User reported in lxc/incus#463 that project
limits were not being enforced on the cluster member in direct
communication with the client during instance creation.

~I also noticed that the limit checks were being skipped based on the
value of the [`User-Agent`
header](d6871e4),
which is spoofable. This pushes the limits check back to the target
cluster member.~

Contains cherry-picks from lxc/incus#505

lxd/instances_post.go

@@ -1238,8 +1238,6 @@ func instancesPost(d *Daemon, r *http.Request) response.Response {
 			if err != nil {
 				return err
 			}
-
-			return nil
 		}
 
 		if !clusterNotification {