Remove Praefect Primary config from docs

What does this MR do?

Removes references to the Praefect configuration field primary from the admin docs. The field was removed from Gitaly.

Part of gitaly#2634 (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