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.

test/h1/config.xml

@@ -24,7 +24,6 @@
         <disableSparseFiles>false</disableSparseFiles>
         <disableTempIndexes>false</disableTempIndexes>
         <paused>false</paused>
-        <weakHashThresholdPct>25</weakHashThresholdPct>
         <markerName>.stfolder</markerName>
         <copyOwnershipFromParent>false</copyOwnershipFromParent>
         <modTimeWindowS>0</modTimeWindowS>
@@ -153,7 +152,6 @@
             <disableSparseFiles>false</disableSparseFiles>
             <disableTempIndexes>false</disableTempIndexes>
             <paused>false</paused>
-            <weakHashThresholdPct>25</weakHashThresholdPct>
             <markerName>.stfolder</markerName>
             <copyOwnershipFromParent>false</copyOwnershipFromParent>
             <modTimeWindowS>0</modTimeWindowS>

test/h2/config.xml

@@ -24,7 +24,6 @@
         <disableSparseFiles>false</disableSparseFiles>
         <disableTempIndexes>false</disableTempIndexes>
         <paused>false</paused>
-        <weakHashThresholdPct>25</weakHashThresholdPct>
         <markerName>.stfolder</markerName>
         <copyOwnershipFromParent>false</copyOwnershipFromParent>
         <modTimeWindowS>0</modTimeWindowS>
@@ -151,7 +150,6 @@
             <disableSparseFiles>false</disableSparseFiles>
             <disableTempIndexes>false</disableTempIndexes>
             <paused>false</paused>
-            <weakHashThresholdPct>25</weakHashThresholdPct>
             <markerName>.stfolder</markerName>
             <copyOwnershipFromParent>false</copyOwnershipFromParent>
             <modTimeWindowS>0</modTimeWindowS>

test/h3/config.xml

@@ -20,7 +20,6 @@
         <disableSparseFiles>false</disableSparseFiles>
         <disableTempIndexes>false</disableTempIndexes>
         <paused>false</paused>
-        <weakHashThresholdPct>25</weakHashThresholdPct>
         <markerName>.stfolder</markerName>
         <copyOwnershipFromParent>false</copyOwnershipFromParent>
         <modTimeWindowS>0</modTimeWindowS>
@@ -50,7 +49,6 @@
         <disableSparseFiles>false</disableSparseFiles>
         <disableTempIndexes>false</disableTempIndexes>
         <paused>false</paused>
-        <weakHashThresholdPct>25</weakHashThresholdPct>
         <markerName>.stfolder</markerName>
         <copyOwnershipFromParent>false</copyOwnershipFromParent>
         <modTimeWindowS>0</modTimeWindowS>

test/h4/config.xml

@@ -17,7 +17,6 @@
         <disableSparseFiles>false</disableSparseFiles>
         <disableTempIndexes>false</disableTempIndexes>
         <paused>false</paused>
-        <weakHashThresholdPct>25</weakHashThresholdPct>
         <markerName>.stfolder</markerName>
         <copyOwnershipFromParent>false</copyOwnershipFromParent>
         <modTimeWindowS>0</modTimeWindowS>