test(integration): revert the workaround to make the latest event integration test pass

[bnjbvr]

Opening this as a draft PR, on top of #6475. When the underlying Synapse bug element-hq/synapse#19713 has been fixed, it will only need a bump of the version of Synapse used in the integration testing (so, in the Dockerfile), and it should start to pass magically.

[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing bnjbvr/integration-test-timeline-limit-after-synapse-bump (3b2c806) with main (0487a14)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.91%. Comparing base (e507eaa) to head (3b2c806).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6476      +/-   ##
==========================================
- Coverage   89.92%   89.91%   -0.02%     
==========================================
  Files         381      381              
  Lines      105402   105402              
  Branches   105402   105402              
==========================================
- Hits        94788    94767      -21     
- Misses       6990     7010      +20     
- Partials     3624     3625       +1     

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