virtual: l1_if.c: Remove unneeded warning message

Commit 5eb17e28ac added this warning when
implementing the full function. However, other backends seem to be also
passing the primitive from the stack to l1sap_up without any visible issues.
Furthermore, according to the documentation of l1sap_up, it takes ownership of the
msgb, but doesn't take ownserhip of the prim itself.

Change-Id: I45fe40e3377eac999d1dac5356678195381d94ca

src/osmo-bts-virtual/l1_if.c

@@ -163,8 +163,7 @@ static void virt_um_rcv_cb(struct virt_um_inst *vui, struct msgb *msg)
 		goto nomessage;
 	}
 
-	/* forward primitive, forwarded msg will not be freed */
-#warning "we cannot just pass a l1sap primitive on the stack!!!"
+	/* forward primitive, lsap takes ownership of the msgb. */
 	l1sap_up(pinst->trx, &l1sap);
 	DEBUGP(DL1P, "Message forwarded to layer 2.\n");
 	return;