Remove ff sidebar_refactor from BE files

What does this MR do?

In this MR we're removing the feature flag sidebar_refactor from the BE files. We can't remove all the references to the ff because there need more changes in views, CSS, ... so it's easier to create several MRs to remove the feature flag.

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 #326111 (closed)