Drop plugins dir support

What does this MR do?

Drop plugins dir support that was deprecated in 13.0. Now only file_hooks dir is supported for executing file hooks.

For #196846 (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