NOTE: This task should be taken on when max-width and search have been implemented (T249363) and deployed for at least 2 weeks across test wikis.
As part of the max-width layout work (T246420), a `layout-max-width.less` file was created containing all the styles relevant to the max-width work along with a `VectorLayoutMaxWidth` feature flag to make the deployment less risky. While the feature flag is helpful in the short-term, it has a high carrying cost  and will be difficult to maintain in the long-term. The max-width design should just be part of the modern Vector experience (non-configurable) and the feature flag removed as soon as possible following its deploy.
Similarly as part of T249363 a feature flag was added for the search in the header feature.
=== Developer Notes
Most of these styles should be easy enough to merge into `layout.less` or `Sidebar.less`, but there are several pending questions:
1) Should the Less variables found in `layout-max-width.less` be moved to `layout.less` or to `variables.less`? I think @Volker_E was of the opinion that these should be part of `variables.less`.
2) Should the styles pertaining only to history and special pages be split into their own stylesheet and conditionally loaded or be part of `layout.less` (where they load on every page)?
3) Should the breakpoint media queries be moved to `layout.less` or is there a better place to put these?
=== Acceptance Criteria
 `VectorLayoutMaxWidth` config flag is removed.
 `VectorIsSearchInHeader` config flag is removed.
 Simplify layout-max-width.less styles, moving into layout.less where applicable.
 Fold layout-search-header.less styles into layout-default, removing and cleaning up styles where possible.
Given the max-width is disabled on certain pages, many of the styles currently found in `layout-max-width.less` and the file itself will need to stay