Geo: Refactor replicables_for_geo_node method

What does this MR do?

This MR is a refactor. No change in behavior.

The aim is to avoid ambiguity and confusion over the intent and usage of this method.

• Removes unused node argument
  • because it is unused.
  • because the current implementations are secondary-specific, but it looks like you could pass in a primary node.
• Renames the method from replicables_for_geo_node to replicables_for_current_secondary
  • because it is more specific (adds more context)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry no behavior change
• 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.