Doxygen: don't extract static members

[VeijoPesonen]

Summary of changes

Static members do not bring any needed information. Those cause only confusion when one is trying to figure what is part of an internal/public API and what is not.

Current Doxygen API documentation

Impact of changes

Migration actions required

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[X] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@AnotherButler
@bulislaw
@evedon

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[ciarmcom]

@VeijoPesonen, thank you for your changes.
@AnotherButler @bulislaw @evedon @ARMmbed/mbed-os-maintainers please review.

[jamesbeyond]

CI Started

[mergify]

This PR does not contain release version label after merging.