[management,client] Default to NetBird's upload service when nothing is configured

The previous commit made a peer with no destination — no MDM override, no URL
named by the caller, nothing published by its management server — refuse to
upload and keep the bundle local unless it was enrolled with NetBird's cloud.
That closed the reported data-boundary concern, but it broke the default for
everyone who uploads a bundle as part of their day: a self-hosted user opening
a support ticket got a refusal where the command used to work.

Product decision (NetBird's, not the reporter's): the knob to keep bundles
inside your own infrastructure is what this branch provides, and it is enough.
The default stays the service NetBird runs, self-hosted included. An admin who
needs the bundles to stay in-house configures the destination; until then the
everyday flow keeps working.

So ResolveUploadURL drops the cloud check, the sentinel error and the
managementURL argument, and never fails:

    MDM  >  explicitly named URL  >  published by management  >  NetBird's service

Nothing observable changes for a deployment that configures nothing, which also
removes two edge cases the fail-closed default had: a peer still enrolled on the
legacy api.wiretrustee.com host would have been classified self-hosted and
refused, and an upgrade would have silently stopped uploads for self-hosted
deployments relying on them. The privilege gate is unaffected — a host other
than the default one still requires a privileged caller, so pointing the CLI
somewhere other than what management published needs root.

client/proto/daemon.pb.go

@@ -2822,10 +2822,8 @@ type DebugBundleRequest struct {
 	AnonymizeLevel string `protobuf:"bytes,8,opt,name=anonymizeLevel,proto3" json:"anonymizeLevel,omitempty"`
 	// upload asks the daemon to upload the bundle. When uploadURL is empty the
 	// daemon resolves the destination itself: the one the management server
-	// publishes, or the service NetBird runs when the peer is enrolled with
-	// NetBird's cloud. A self-hosted deployment that publishes no destination
-	// gets no upload, so a bundle never leaves the operator's control sphere by
-	// default. uploadURL still overrides it, for a privileged caller.
+	// publishes, or else the service NetBird runs. uploadURL still overrides it,
+	// and a host other than the default one requires a privileged caller.
 	Upload        bool `protobuf:"varint,9,opt,name=upload,proto3" json:"upload,omitempty"`
 	unknownFields protoimpl.UnknownFields
 	sizeCache     protoimpl.SizeCache