Clean up dead code guarded by outdated oneMKL and DPC++ version checks

[antonwolfy]

Since the build and runtime minimum was raised to oneMKL / DPC++ compiler 2026.0 (conda-recipe/meta.yaml), several preprocessor and version guards now select a fixed branch on every supported configuration. This PR removes the resulting dead code and replaces version comparisons with the condition they actually encode.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2999/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev3=py313h509198e_4 ran successfully.
Passed: 1373
Failed: 2
Skipped: 5