Remove superfluous call (amend bd339713)

Graph::prep is a private function called initially from the main
process-callback before doing any work, and later at the end of
each process cycle. This sets up the process graph-chain and
process trigger queue.

Tempo information is irrelevant here.

libs/ardour/graph.cc

@@ -227,8 +227,6 @@ Graph::prep ()
 		g_atomic_int_inc (&_trigger_queue_size);
 		_trigger_queue.push_back (i.get ());
 	}
-
-	Temporal::TempoMap::fetch();
 }
 
 void