Remove non-core adapters

[reinecke]

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This PR covers the major work of #1386.

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Summarize your change.

This PR applies core repo cleanup after creating separate repos for adapters in the OpenTimelineIO GitHub org.

• Removes contrib adapters and tree
• Removes all but the "native" OTIO adapters from py-opentimelineio/opentimelineio/adapters
• Updates to plugin system and documentation to support adapter removal

This PR is the result of contributions from the broad community including:

• @apetrynet
• @markreidvfx
• @rosborne132
• @timlehr

[reinecke]

Build failing due to scikit-build/cmake-python-distributions/pull/490