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.

proto/bep/bep.proto

@@ -145,7 +145,7 @@ message BlockInfo {
   bytes hash = 3;
   int64 offset = 1;
   int32 size = 2;
-  uint32 weak_hash = 4;
+  reserved 4;
 }
 
 message Vector {
@@ -203,8 +203,8 @@ message Request {
   int32 size = 5;
   bytes hash = 6;
   bool from_temporary = 7;
-  uint32 weak_hash = 8;
   int32 block_no = 9;
+  reserved 8;
 }
 
 // Response