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{},
 				},

lib/config/folderconfiguration.go

@@ -70,7 +70,6 @@ type FolderConfiguration struct {
 	DisableSparseFiles      bool                        `json:"disableSparseFiles" xml:"disableSparseFiles"`
 	DisableTempIndexes      bool                        `json:"disableTempIndexes" xml:"disableTempIndexes"`
 	Paused                  bool                        `json:"paused" xml:"paused"`
-	WeakHashThresholdPct    int                         `json:"weakHashThresholdPct" xml:"weakHashThresholdPct"`
 	MarkerName              string                      `json:"markerName" xml:"markerName"`
 	CopyOwnershipFromParent bool                        `json:"copyOwnershipFromParent" xml:"copyOwnershipFromParent"`
 	RawModTimeWindowS       int                         `json:"modTimeWindowS" xml:"modTimeWindowS"`
@@ -311,10 +310,6 @@ func (f *FolderConfiguration) prepare(myID protocol.DeviceID, existingDevices ma
 		f.Versioning.CleanupIntervalS = 0
 	}
 
-	if f.WeakHashThresholdPct == 0 {
-		f.WeakHashThresholdPct = 25
-	}
-
 	if f.MarkerName == "" {
 		f.MarkerName = DefaultMarkerName
 	}

lib/config/testdata/overridenvalues.xml

@@ -70,7 +70,6 @@
             <disableSparseFiles>false</disableSparseFiles>
             <disableTempIndexes>false</disableTempIndexes>
             <paused>false</paused>
-            <weakHashThresholdPct>25</weakHashThresholdPct>
             <markerName>.stfolder</markerName>
             <copyOwnershipFromParent>false</copyOwnershipFromParent>
             <modTimeWindowS>0</modTimeWindowS>