Move OpenMP to swmm-solver

[michaeltryby]

This pull request removes support for building the OpenMP library on Apple platforms from the SWMM Toolkit build system. The changes simplify the build process by eliminating the custom OpenMP build logic and its associated license file, and update references and dependencies accordingly.

Build system simplification:

• Removed Apple-specific logic for building the OpenMP library from the main CMakeLists.txt, so OpenMP is now always expected to be provided by the system or found via CMake. (swmm-toolkit/CMakeLists.txt)
• Removed the custom CMake module for fetching and building the OpenMP library on Darwin (swmm-toolkit/extern/openmp.cmake).
• Removed the OpenMP license file that was associated with the custom build process (swmm-toolkit/extern/license-llvm-openmp.txt).
• Updated the solver target to remove conditional linking to the custom OpenMP target, further simplifying the build logic (swmm-toolkit/src/swmm/toolkit/CMakeLists.txt).

General maintenance:

• Updated project metadata comments in CMakeLists.txt files to reflect the latest modification date. (swmm-toolkit/CMakeLists.txt, swmm-toolkit/src/swmm/toolkit/CMakeLists.txt) [1] [2]
• Updated the submodule reference for swmm-solver to a newer commit. (swmm-toolkit/swmm-solver)

[michaeltryby]

Hey @karosc, I confirmed the OpenMP linkage in libswmm5 packaged in the wheel. Thanks for showing me how to do it!

[karosc]

All around nice cleanup. I particularly like the library alias and interface aspect of it all. Confirmed the link is working on my end for all platforms.

👏🙏🙇‍♂️