feat(catalog): remove skill catalog endpoints from v1alpha1 API

[pboyd]

Description

Since the skill catalog has never been released as part of the v1alpha API, remove it from there so it's only available as part of the new v1 endpoints.

How Has This Been Tested?

Verified that the skill catalog endpoints return 404 on v1alpha1 and work on v1.

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

• The commits have meaningful messages
• Automated tests are provided as part of the PR for major new functionalities; testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work.
• Code changes follow the kubeflow contribution guidelines.

[google-oss-prow]

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