Improve deletion of timer

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 82.65%. Comparing base (a1e87b0) to head (c2b052e).

Files with missing lines	Patch %	Lines
src/dxtb/_src/timing/timer.py	52.63%	9 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #176 +/- ## ========================================== - Coverage 82.72% 82.65% -0.07% ========================================== Files 195 195 Lines 9717 9732 +15 Branches 1473 1475 +2 ========================================== + Hits 8038 8044 +6 - Misses 1328 1337 +9 Partials 351 351 ``` | [Flag](https://app.codecov.io/gh/grimme-lab/dxtb/pull/176/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=grimme-lab) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/grimme-lab/dxtb/pull/176/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=grimme-lab) | `82.65% <55.00%> (-0.07%)` | :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=grimme-lab#carryforward-flags-in-the-pull-request-comment) to find out more.

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