Leave only onemain landmark on files page

[JuliaKirschenheuter]

There should be only one main landmark on the page. If there are any reasons to have several then we have to differentiate between them: https://www.w3.org/WAI/ARIA/apg/patterns/landmarks/examples/main.html

In my eyes there is no reason to have several main landmarks for files page. On all other pages (which are migrated to vue) there is only app-content-vue main landmark, like:

Actually <main id="content" class="app-files"> should not exist because files page is already migrated to vue.

[ShGKme]

Waiting for the backport

[JuliaKirschenheuter]

closing as done