Draft: Remove redundant external wiki methods from Project model

Draft status

This MR is branched from !50916 (merged), and will remain a Draft until that branch is merged to master, as it relies on the projects.has_external_wiki column being NOT NULL.

What does this MR do?

This MR removes two methods from the Project model to do with the "external wiki" service as they are no longer needed.

Project#external_wiki? is no longer called after !49916 (merged) removed the only calling code.

Project#external_wiki can be removed and instead we can use the existing has_one association #external_wiki_service (see: !49916 (comment 482585620)).

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done