Remove unused PhysConst

[AlexanderSinn]

#1311 left some unused variables.

• 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

[lucafedeli88]

Ah, curious that the compiler does not find the issue... When I tried implementing a new clang-tidy-based CI test, clang flagged these two variables as unused (see #1303) !

[AlexanderSinn]

The compilers from the CI tests do catch it, except nvcc which I also use locally and didn't see it. However, they don't have warnings as errors enabled.