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/protocol/common_test.go

@@ -17,7 +17,6 @@ type TestModel struct {
 	offset        int64
 	size          int32
 	hash          []byte
-	weakHash      uint32
 	fromTemporary bool
 	indexFn       func(string, []FileInfo)
 	ccFn          func(*ClusterConfig)
@@ -48,7 +47,6 @@ func (t *TestModel) Request(_ Connection, req *Request) (RequestResponse, error)
 	t.offset = req.Offset
 	t.size = int32(req.Size)
 	t.hash = req.Hash
-	t.weakHash = req.WeakHash
 	t.fromTemporary = req.FromTemporary
 	buf := make([]byte, len(t.data))
 	copy(buf, t.data)