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/config/config_test.go

@@ -114,10 +114,9 @@ func TestDefaultValues(t *testing.T) {
 					CleanupIntervalS: 3600,
 					Params:           map[string]string{},
 				},
-				MaxConflicts:         10,
-				WeakHashThresholdPct: 25,
-				MarkerName:           ".stfolder",
-				MaxConcurrentWrites:  2,
+				MaxConflicts:        10,
+				MarkerName:          ".stfolder",
+				MaxConcurrentWrites: 2,
 				XattrFilter: XattrFilter{
 					Entries:            []XattrFilterEntry{},
 					MaxSingleEntrySize: 1024,
@@ -190,10 +189,9 @@ func TestDeviceConfig(t *testing.T) {
 				Versioning: VersioningConfiguration{
 					Params: map[string]string{},
 				},
-				WeakHashThresholdPct: 25,
-				MarkerName:           DefaultMarkerName,
-				JunctionsAsDirs:      true,
-				MaxConcurrentWrites:  maxConcurrentWritesDefault,
+				MarkerName:          DefaultMarkerName,
+				JunctionsAsDirs:     true,
+				MaxConcurrentWrites: maxConcurrentWritesDefault,
 				XattrFilter: XattrFilter{
 					Entries: []XattrFilterEntry{},
 				},