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/model/fakeconns_test.go

@@ -75,7 +75,7 @@ func (f *fakeConnection) DownloadProgress(_ context.Context, dp *protocol.Downlo
 
 func (f *fakeConnection) addFileLocked(name string, flags uint32, ftype protocol.FileInfoType, data []byte, version protocol.Vector, localFlags uint32) {
 	blockSize := protocol.BlockSize(int64(len(data)))
-	blocks, _ := scanner.Blocks(context.TODO(), bytes.NewReader(data), blockSize, int64(len(data)), nil, true)
+	blocks, _ := scanner.Blocks(context.TODO(), bytes.NewReader(data), blockSize, int64(len(data)), nil)
 
 	file := protocol.FileInfo{
 		Name:       name,