Remove amrex::Gpu::notInLaunchRegion() checks

[AlexanderSinn]

The launch region check is a way in amrex to easily disable GPU functionality in parts of the code that are not fully ported to GPU yet. HiPACE++ is fully ported to GPU and doesn't use the launch region, especially not when compiling with OMP, so the checks can be removed to clean the code.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[MaxThevenet]

Thanks for this PR!