Remove duplicate find_package(Python3) call

[scpeters]

🦟 Bug fix

Part of https://github.com/gazebosim/gz-sim/issues/2249.

Summary

There is already a find_package(Python3) call on line 124 next to the other python logic, so remove the earlier duplicate call, as there is nothing else that needs it that early.

Checklist

• [X] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [X] codecheck passed (See contributing)
• [X] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov[bot]]

Codecov Report

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

Comparison is base (daeb089) 97.67% compared to head (05f5139) 97.67%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## gz-msgs10 #420 +/- ## ========================================== Coverage 97.67% 97.67% ========================================== Files 27 27 Lines 1163 1163 ========================================== Hits 1136 1136 Misses 27 27 ```

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