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/api/testdata/config/config.xml

@@ -18,7 +18,6 @@
         <disableSparseFiles>false</disableSparseFiles>
         <disableTempIndexes>false</disableTempIndexes>
         <paused>false</paused>
-        <weakHashThresholdPct>25</weakHashThresholdPct>
         <markerName>.stfolder</markerName>
         <useLargeBlocks>true</useLargeBlocks>
     </folder>
@@ -39,7 +38,6 @@
         <disableSparseFiles>false</disableSparseFiles>
         <disableTempIndexes>false</disableTempIndexes>
         <paused>false</paused>
-        <weakHashThresholdPct>25</weakHashThresholdPct>
         <markerName>.stfolder</markerName>
         <useLargeBlocks>true</useLargeBlocks>
     </folder>