Clean up callback conditions

[efaulhaber]

As discussed with @ranocha, the condition integrator.stats.naccept == 0 && integrator.iter > 0 is an historical remnant that has just been copied around without really thinking about it.

The callbacks are only triggered for accepted steps, so this condition is never true. The callback will only be initially triggered once with naccept == iter == 0 and then always with iter >= naccept > 0.

[github-actions[bot]]

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.53%. Comparing base (5359525) to head (7d64acc). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1960 +/- ## ========================================== - Coverage 96.11% 89.53% -6.58% ========================================== Files 460 460 Lines 36926 36915 -11 ========================================== - Hits 35490 33049 -2441 - Misses 1436 3866 +2430 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1960/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1960/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `89.53% <100.00%> (-6.58%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

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