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/syncthing/internals.go

@@ -42,7 +42,7 @@ func (m *Internals) SetIgnores(folderID string, content []string) error {
 }
 
 func (m *Internals) DownloadBlock(ctx context.Context, deviceID protocol.DeviceID, folderID string, path string, blockNumber int, blockInfo protocol.BlockInfo, allowFromTemporary bool) ([]byte, error) {
-	return m.model.RequestGlobal(ctx, deviceID, folderID, path, int(blockNumber), blockInfo.Offset, blockInfo.Size, blockInfo.Hash, blockInfo.WeakHash, allowFromTemporary)
+	return m.model.RequestGlobal(ctx, deviceID, folderID, path, int(blockNumber), blockInfo.Offset, blockInfo.Size, blockInfo.Hash, allowFromTemporary)
 }
 
 func (m *Internals) BlockAvailability(folderID string, file protocol.FileInfo, block protocol.BlockInfo) ([]model.Availability, error) {

lib/syncthing/syncthing.go

@@ -177,7 +177,7 @@ func (a *App) startup() error {
 		}()
 	}
 
-	perf := ur.CpuBench(context.Background(), 3, 150*time.Millisecond, true)
+	perf := ur.CpuBench(context.Background(), 3, 150*time.Millisecond)
 	l.Infof("Hashing performance is %.02f MB/s", perf)
 
 	if err := db.UpdateSchema(a.ll); err != nil {