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/sharedpullerstate.go

@@ -285,15 +285,6 @@ func (s *sharedPullerState) skippedSparseBlock(bytes int) {
 	metricFolderProcessedBytesTotal.WithLabelValues(s.folder, metricSourceSkipped).Add(float64(bytes))
 }
 
-func (s *sharedPullerState) copiedFromOriginShifted(bytes int) {
-	s.mut.Lock()
-	s.copyOrigin++
-	s.copyOriginShifted++
-	s.updated = time.Now()
-	s.mut.Unlock()
-	metricFolderProcessedBytesTotal.WithLabelValues(s.folder, metricSourceLocalShifted).Add(float64(bytes))
-}
-
 func (s *sharedPullerState) pullStarted() {
 	s.mut.Lock()
 	s.copyTotal--