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/blockqueue.go

@@ -16,7 +16,7 @@ import (
 )
 
 // HashFile hashes the files and returns a list of blocks representing the file.
-func HashFile(ctx context.Context, folderID string, fs fs.Filesystem, path string, blockSize int, counter Counter, useWeakHashes bool) ([]protocol.BlockInfo, error) {
+func HashFile(ctx context.Context, folderID string, fs fs.Filesystem, path string, blockSize int, counter Counter) ([]protocol.BlockInfo, error) {
 	fd, err := fs.Open(path)
 	if err != nil {
 		l.Debugln("open:", err)
@@ -36,7 +36,7 @@ func HashFile(ctx context.Context, folderID string, fs fs.Filesystem, path strin
 
 	// Hash the file. This may take a while for large files.
 
-	blocks, err := Blocks(ctx, fd, blockSize, size, counter, useWeakHashes)
+	blocks, err := Blocks(ctx, fd, blockSize, size, counter)
 	if err != nil {
 		l.Debugln("blocks:", err)
 		return nil, err
@@ -108,7 +108,7 @@ func (ph *parallelHasher) hashFiles(ctx context.Context) {
 				panic("Bug. Asked to hash a directory or a deleted file.")
 			}
 
-			blocks, err := HashFile(ctx, ph.folderID, ph.fs, f.Name, f.BlockSize(), ph.counter, true)
+			blocks, err := HashFile(ctx, ph.folderID, ph.fs, f.Name, f.BlockSize(), ph.counter)
 			if err != nil {
 				handleError(ctx, "hashing", f.Name, err, ph.outbox)
 				continue