Cleanup - Removes LoadedProgram::maybe_expiration_slot

[Lichtso]

Problem

When switching from explicit to implicit DelayVisibility tombstones, we have forgotten to remove their expiration mechanism which became unreachable:
During extraction the DelayVisibility tombstone is created implicitly and inserted into the batch cache as a hit. Because it does not count as a miss, it is never loaded and assigned / replenished. Thus, the DelayVisibility tombstones only exists on the TX batch level, not on the global loaded programs cache. So, they can never be encountered during extract() and don't need to be skipped. And they can also not be pruned by expiration as they never get inserted in the first place.

Summary of Changes

• Removes LoadedProgram::maybe_expiration_slot
• Removes Stats::prunes_expired
• Removes the logic that skips DelayVisibility tombstones in LoadedPrograms::is_entry_usable()
• Removes the logic that prunes DelayVisibility tombstones in LoadedPrograms::prune()
• Removes related tests

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (fddfc84) 81.6% compared to head (e62a1e8) 81.6%.
Report is 1 commits behind head on master.

❗ Current head e62a1e8 differs from pull request most recent head 6528ce9. Consider uploading reports for the commit 6528ce9 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #35023     +/-   ##
=========================================
- Coverage    81.6%    81.6%   -0.1%     
=========================================
  Files         830      830             
  Lines      225031   224844    -187     
=========================================
- Hits       183748   183584    -164     
+ Misses      41283    41260     -23