eth/downloader: purge pre-merge sync code (ethereum#29281)

This PR removes pre-merge sync logic from the downloader. Now-irrelevant tests are removed and others have been updated.

eth/downloader/beaconsync.go

@@ -106,7 +106,7 @@ func (b *beaconBackfiller) resume() {
 		}()
 		// If the downloader fails, report an error as in beacon chain mode there
 		// should be no errors as long as the chain we're syncing to is valid.
-		if err := b.downloader.synchronise("", common.Hash{}, nil, nil, mode, true, b.started); err != nil {
+		if err := b.downloader.synchronise(mode, b.started); err != nil {
 			log.Error("Beacon backfilling failed", "err", err)
 			return
 		}
@@ -268,9 +268,9 @@ func (d *Downloader) findBeaconAncestor() (uint64, error) {
 	return start, nil
 }
 
-// fetchBeaconHeaders feeds skeleton headers to the downloader queue for scheduling
+// fetchHeaders feeds skeleton headers to the downloader queue for scheduling
 // until sync errors or is finished.
-func (d *Downloader) fetchBeaconHeaders(from uint64) error {
+func (d *Downloader) fetchHeaders(from uint64) error {
 	var head *types.Header
 	_, tail, _, err := d.skeleton.Bounds()
 	if err != nil {