core: drop another useless genesis-specific header handling branch

If we only have genesis block (or <2000 headers) then we might as well use
generic logic below with zero targetHash because genesis block has zero
PrevHash (and its hash will naturally be the last on the chain going
backwards).

pkg/core/blockchain.go

@@ -434,13 +434,6 @@ func (bc *Blockchain) init() error {
 		var targetHash util.Uint256
 		if len(bc.headerHashes) > 0 {
 			targetHash = bc.headerHashes[len(bc.headerHashes)-1]
-		} else {
-			genesisBlock, err := CreateGenesisBlock(bc.config)
-			if err != nil {
-				return err
-			}
-			targetHash = genesisBlock.Hash()
-			bc.headerHashes = append(bc.headerHashes, targetHash)
 		}
 		headers := make([]util.Uint256, 0, headerBatchCount)