Remove KOKKOS_MUPHYS scheme and retain only AES_GRAUPEL in muphys module

[edopao]

This PR removes the legacy KOKKOS_MUPHYS scheme from the muphys microphysics module, making AES_GRAUPEL the sole remaining scheme. Changes include:

• Deletion of MuphysScheme enum and related configuration
• Removal of use_aes_graupel flag and conditional branching throughout the codebase
• Deletion of muphys_wrapper.py (Fortran binding for graupel-only driver)
• Removal of dependency on icon4py-atmosphere-muphys in bindings/pyproject.toml
• Cleanup of driver setup functions that previously accepted scheme parameters
• Deletion of redundant test files (integration and stencil tests) specific to KOKKOS_MUPHYS
• Updates to driver and test references to reflect scheme removal

The AES_GRAUPEL scheme (based on ICON's mo_aes_graupel.f90) is now the only microphysics implementation, simplifying maintenance and reducing code complexity.

[havogt]

I think we should discuss if we keep them (we could great a new reference from the icon4py implementation). Briefly discussed this with Jacopo, but forgot what the conclusion was. We should define what the muphys component in icon4py should be, the one that is tested in these tests or the one that returns tendencies...

[github-actions]

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