discovery/sync_manager: remove unneeded markGraphSyncing

AFAICT it's not possible to flip back from bein synced_to_chain, so we
remove the underlying call that could reflect this. The method is moved
into the test file since it's still used to test correctness of other
portions of the flow.

discovery/sync_manager.go

@@ -316,7 +316,6 @@ func (m *SyncManager) syncerHandler() {
 			if !attemptHistoricalSync {
 				continue
 			}
-			m.markGraphSyncing()
 
 			log.Debugf("Attempting initial historical sync with "+
 				"GossipSyncer(%x)", s.cfg.peerPub)
@@ -751,12 +750,6 @@ func (m *SyncManager) markGraphSynced() {
 	atomic.StoreInt32(&m.initialHistoricalSyncCompleted, 1)
 }
 
-// markGraphSyncing allows us to report that the initial historical sync is
-// still undergoing.
-func (m *SyncManager) markGraphSyncing() {
-	atomic.StoreInt32(&m.initialHistoricalSyncCompleted, 0)
-}
-
 // IsGraphSynced determines whether we've completed our initial historical sync.
 // The initial historical sync is done to ensure we've ingested as much of the
 // public graph as possible.