consensus/clique, core: API cleanup (#23100)

This removes some code:

- The clique engine calculated the snapshot twice when verifying headers/blocks.

- The method GetBlockHashesFromHash in Header/Block/Lightchain was only used by tests. It
  is now removed from the API.
  
- The method GetTdByHash internally looked up the number before calling GetTd(hash, num).
  In many cases, callers already had the number, and used this method just because it has a
  shorter name. I have removed the method to make the API surface smaller.

eth/protocols/eth/handler_test.go

@@ -126,6 +126,12 @@ func testGetBlockHeaders(t *testing.T, protocol uint) {
 	for i := range unknown {
 		unknown[i] = byte(i)
 	}
+	getHashes := func(from, limit uint64) (hashes []common.Hash) {
+		for i := uint64(0); i < limit; i++ {
+			hashes = append(hashes, backend.chain.GetCanonicalHash(from-1-i))
+		}
+		return hashes
+	}
 	// Create a batch of tests for various scenarios
 	limit := uint64(maxHeadersServe)
 	tests := []struct {
@@ -183,7 +189,7 @@ func testGetBlockHeaders(t *testing.T, protocol uint) {
 		// Ensure protocol limits are honored
 		{
 			&GetBlockHeadersPacket{Origin: HashOrNumber{Number: backend.chain.CurrentBlock().NumberU64() - 1}, Amount: limit + 10, Reverse: true},
-			backend.chain.GetBlockHashesFromHash(backend.chain.CurrentBlock().Hash(), limit),
+			getHashes(backend.chain.CurrentBlock().NumberU64(), limit),
 		},
 		// Check that requesting more than available is handled gracefully
 		{