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.

cmd/syncthing/decrypt/decrypt.go

@@ -238,7 +238,7 @@ func (c *CLI) decryptFile(encFi *protocol.FileInfo, plainFi *protocol.FileInfo,
 		}
 
 		// Verify the hash against the plaintext block info
-		if !scanner.Validate(dec, plainBlock.Hash, 0) {
+		if !scanner.Validate(dec, plainBlock.Hash) {
 			// The block decrypted correctly but fails the hash check. This
 			// is odd and unexpected, but it it's still a valid block from
 			// the source. The file might have changed while we pulled it?