remove todo Elterngeld

[ChristianZimpelmann]

What problem do you want to solve?

Closes #605

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (4aeca1a) 90.53% compared to head (ee1ae6d) 90.53%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #646 +/- ## ======================================= Coverage 90.53% 90.53% ======================================= Files 48 48 Lines 3095 3095 ======================================= Hits 2802 2802 Misses 293 293 ``` | [Files Changed](https://app.codecov.io/gh/iza-institute-of-labor-economics/gettsim/pull/646?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iza-institute-of-labor-economics) | Coverage Δ | | |---|---|---| | [src/\_gettsim/transfers/elterngeld.py](https://app.codecov.io/gh/iza-institute-of-labor-economics/gettsim/pull/646?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iza-institute-of-labor-economics#diff-c3JjL19nZXR0c2ltL3RyYW5zZmVycy9lbHRlcm5nZWxkLnB5) | `100.00% <ø> (ø)` | |

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