chore: remove weak hashing which does not pull its weight (#10005)

We've had weak/rolling hashing in the code for quite a while. It was a
popular request for a while, based on the belief that rsync does this
and we should too. However, the benefit is quite small; we save on
average about 0.8% of transferred blocks over the population as a whole:

<img width="974" alt="Screenshot 2025-03-28 at 17 09 02"
src="https://github.com/user-attachments/assets/bbe10dea-f85e-4043-9823-7cef1220b4a2"
/>

This would be fine if the cost was comparably low, however the downside
of attempting rolling hash matching is that we (by default) do a
complete file read on the destination in order to look for matches
before we starting pulling blocks for the file. For any larger file this
means a sometimes long, I/O-intensive pause before the file starts
syncing, for usually no benefit.

I propose we simply rip off the bandaid and save the effort.

lib/scanner/blocks.go

@@ -10,8 +10,6 @@ import (
 	"bytes"
 	"context"
 	"crypto/sha256"
-	"hash"
-	"hash/adler32"
 	"io"
 
 	"github.com/syncthing/syncthing/lib/protocol"
@@ -24,7 +22,7 @@ type Counter interface {
 }
 
 // Blocks returns the blockwise hash of the reader.
-func Blocks(ctx context.Context, r io.Reader, blocksize int, sizehint int64, counter Counter, useWeakHashes bool) ([]protocol.BlockInfo, error) {
+func Blocks(ctx context.Context, r io.Reader, blocksize int, sizehint int64, counter Counter) ([]protocol.BlockInfo, error) {
 	if counter == nil {
 		counter = &noopCounter{}
 	}
@@ -32,15 +30,6 @@ func Blocks(ctx context.Context, r io.Reader, blocksize int, sizehint int64, cou
 	hf := sha256.New()
 	const hashLength = sha256.Size
 
-	var weakHf hash.Hash32 = noopHash{}
-	var multiHf io.Writer = hf
-	if useWeakHashes {
-		// Use an actual weak hash function, make the multiHf
-		// write to both hash functions.
-		weakHf = adler32.New()
-		multiHf = io.MultiWriter(hf, weakHf)
-	}
-
 	var blocks []protocol.BlockInfo
 	var hashes, thisHash []byte
 
@@ -70,7 +59,7 @@ func Blocks(ctx context.Context, r io.Reader, blocksize int, sizehint int64, cou
 		}
 
 		lr.N = int64(blocksize)
-		n, err := io.CopyBuffer(multiHf, lr, buf)
+		n, err := io.CopyBuffer(hf, lr, buf)
 		if err != nil {
 			return nil, err
 		}
@@ -87,17 +76,15 @@ func Blocks(ctx context.Context, r io.Reader, blocksize int, sizehint int64, cou
 		thisHash, hashes = hashes[:hashLength], hashes[hashLength:]
 
 		b := protocol.BlockInfo{
-			Size:     int(n),
-			Offset:   offset,
-			Hash:     thisHash,
-			WeakHash: weakHf.Sum32(),
+			Size:   int(n),
+			Offset: offset,
+			Hash:   thisHash,
 		}
 
 		blocks = append(blocks, b)
 		offset += n
 
 		hf.Reset()
-		weakHf.Reset()
 	}
 
 	if len(blocks) == 0 {
@@ -112,14 +99,8 @@ func Blocks(ctx context.Context, r io.Reader, blocksize int, sizehint int64, cou
 	return blocks, nil
 }
 
-// Validate quickly validates buf against the 32-bit weakHash, if not zero,
-// else against the cryptohash hash, if len(hash)>0. It is satisfied if
-// either hash matches or neither hash is given.
-func Validate(buf, hash []byte, weakHash uint32) bool {
-	if weakHash != 0 && adler32.Checksum(buf) == weakHash {
-		return true
-	}
-
+// Validate validates the hash, if len(hash)>0.
+func Validate(buf, hash []byte) bool {
 	if len(hash) > 0 {
 		hbuf := sha256.Sum256(buf)
 		return bytes.Equal(hbuf[:], hash)