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/requests_test.go

@@ -436,11 +436,8 @@ func TestRescanIfHaveInvalidContent(t *testing.T) {
 	}
 
 	f := checkReceived(<-received)
-	if f.Blocks[0].WeakHash != 103547413 {
-		t.Fatalf("unexpected weak hash: %d != 103547413", f.Blocks[0].WeakHash)
-	}
 
-	res, err := m.Request(device1Conn, &protocol.Request{Folder: "default", Name: "foo", Size: len(payload), Hash: f.Blocks[0].Hash, WeakHash: f.Blocks[0].WeakHash})
+	res, err := m.Request(device1Conn, &protocol.Request{Folder: "default", Name: "foo", Size: len(payload), Hash: f.Blocks[0].Hash})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -454,17 +451,14 @@ func TestRescanIfHaveInvalidContent(t *testing.T) {
 
 	writeFile(t, tfs, "foo", payload)
 
-	_, err = m.Request(device1Conn, &protocol.Request{Folder: "default", Name: "foo", Size: len(payload), Hash: f.Blocks[0].Hash, WeakHash: f.Blocks[0].WeakHash})
+	_, err = m.Request(device1Conn, &protocol.Request{Folder: "default", Name: "foo", Size: len(payload), Hash: f.Blocks[0].Hash})
 	if err == nil {
 		t.Fatalf("expected failure")
 	}
 
 	select {
 	case fs := <-received:
-		f := checkReceived(fs)
-		if f.Blocks[0].WeakHash != 41943361 {
-			t.Fatalf("unexpected weak hash: %d != 41943361", f.Blocks[0].WeakHash)
-		}
+		checkReceived(fs)
 	case <-time.After(time.Second):
 		t.Fatalf("timed out")
 	}