Remove flaky prepost exp hook test

[yngve-sk]

Resolves #9671
Not sure of the underlying cause for the flakiness, removing as it is already covered by test_model_hook_order

[codspeed-hq]

CodSpeed Performance Report

Merging #9785 will degrade performances by 10.68%

_{Comparing yngve-sk:25.01.17.mark-prepost-exp-hook-test-as-flaky (6720aab) with main (faf3d12)}

Summary

❌ 1 regressions
✅ 23 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	yngve-sk:25.01.17.mark-prepost-exp-hook-test-as-flaky	Change
❌	test_direct_dark_performance_with_storage[gen_x: 20, sum_x: 20 reals: 10-summary-get_record_observations]	1.3 ms	1.4 ms	-10.68%

[oyvindeide]

Nice! Could you just write in the commit body that this is already covered by test_hook_order?