Remove destroy_personal_snippet ability

What does this MR do?

Removes the destroy_personal_snippet ability since it's only used (incorrectly) by the snippet API, and use the admin_personal_snippet ability instead (since it's the one used everywhere else)

Refs #37254 (closed)

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