Fix hanging P4est MPI simulation with boundary conditions

[benegee]

Removing the if n_boundaries > 0 check fixes #1878.

Nothing will be done on the rank with 0 boundaries but p4est_iterate and ultimately MPI AllReduce will be called by all ranks.

The other checks for the mortars seem to be likewise at risk to me.

[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 96.30%. Comparing base (909abb4) to head (feeb716).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1879 +/- ## ========================================== - Coverage 96.30% 96.30% -0.00% ========================================== Files 440 440 Lines 35793 35790 -3 ========================================== - Hits 34470 34467 -3 Misses 1323 1323 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1879/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/1879/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `96.30% <ø> (-<0.01%)` | :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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