Resolve "Hide the variables menu and the visibility tab"

What is in this MR

In this MR we de-register the VariablesPageHeaderItemType and VisibilityPageSidePanelType registrables. Both are placeholders for features which don't currently exist.

How to test this MR

• In the nav confirm that Variables is now gone.
• In the sidebar confirm that Visiblity is now gone.

Merge Request Checklist

• changelog.md has been updated if required.
• New/updated Premium/Enterprise features are separated correctly in the premium or enterprise folder
• The latest Chrome and Firefox have been used to test any new frontend features
• Documentation has been updated
• Quality Standards are met
• Performance: tables are still fast with 100k+ rows, 100+ field tables
• The redoc API pages have been updated for any REST API changes
• Our custom API docs are updated for changes to endpoints accessed via api tokens
• The UI/UX has been updated following UI Style Guide

Closes #2218 (closed)