stream-service: move CAP_CHOWN check from plugins to service constructor

A plugin service can be a TCP socket now, so it does not make much sense
to strictly check for CAP_CHOWN.

src/libcharon/plugins/error_notify/error_notify_plugin.c

@@ -92,12 +92,6 @@ plugin_t *error_notify_plugin_create()
 {
 	private_error_notify_plugin_t *this;
 
-	if (!lib->caps->check(lib->caps, CAP_CHOWN))
-	{	/* required to chown(2) notify socket */
-		DBG1(DBG_CFG, "error-notify plugin requires CAP_CHOWN capability");
-		return NULL;
-	}
-
 	INIT(this,
 		.public = {
 			.plugin = {