remove some minor allocations for threaded FDSBP

[ranocha]

I have observed some performance problems with FDSBP solvers and multiple threads on Julia 1.9. It seems to be okay again in Julia 1.10. So far, I haven't been able to figure out what exactly is wrong there. While debugging this stuff, I noticed a few minor performance improvements avoiding some allocations with multiple threads.

[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 (1ca37cf) to head (40f8d25). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1868 +/- ## ========================================== + Coverage 89.44% 96.30% +6.85% ========================================== Files 438 438 Lines 35593 35608 +15 ========================================== + Hits 31836 34289 +2453 + Misses 3757 1319 -2438 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1868/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/1868/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `96.30% <100.00%> (+6.85%)` | :arrow_up: | 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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