Remove ES parent relation for project snippets

What does this MR do?

Ref: #34558 (closed)

!18459 (merged) added a join relation on project snippets to support searching public/internal projects.

Since this requires recreating the index this broke indexing on existing indices, so we decided to revert this change for now.

We're keeping most of the changes from the original MR since it also refactored the ES snippet classes to align with the other models.

We plan to revert these changes again at a later date, this is tracked in #35760 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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