Wiki repositories use default branch when creating the repo

What does this MR do?

In this MR we're removing the feature flag wiki_uses_default_branch. This will allow wiki repositories to use the instance default branch when creating the repo. If no default branch is set, we will default to the code default ref which, at this point, is main.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #334754 (closed)