CMake: Simplify ABACUS executable naming logic

[Growl1234]

The previous implementation enumerated most combinations of LCAO/PW, LibRI, ML, CUDA, and MPI through deeply nested conditionals. This PR instead determines the executable name from two independent components:

• feature: pw, basic, std, ml, or max;
• backend: gpu, para, omp, or ser.

The final name is then constructed as abacus_<feature>_<backend>, like what's provided before.

Changes

• Remove the duplicated nested executable-naming branches.
• Treat CUDA, ROCm, and CUDA-on-DCU builds consistently as gpu.
• Distinguish non-MPI OpenMP builds with the omp suffix instead of classifying them as ser.
• Ensure the executable name is always determined without relying on particular MPI/GPU combinations.
• Remove the unnecessary cache-variable cleanup for ABACUS_BIN_NAME.

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