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/bep_request_response.go

@@ -25,7 +25,6 @@ type Request struct {
 	Size          int
 	Hash          []byte
 	FromTemporary bool
-	WeakHash      uint32
 	BlockNo       int
 }
 
@@ -38,7 +37,6 @@ func (r *Request) toWire() *bep.Request {
 		Size:          int32(r.Size),
 		Hash:          r.Hash,
 		FromTemporary: r.FromTemporary,
-		WeakHash:      r.WeakHash,
 		BlockNo:       int32(r.BlockNo),
 	}
 }
@@ -52,7 +50,6 @@ func requestFromWire(w *bep.Request) *Request {
 		Size:          int(w.Size),
 		Hash:          w.Hash,
 		FromTemporary: w.FromTemporary,
-		WeakHash:      w.WeakHash,
 		BlockNo:       int(w.BlockNo),
 	}
 }