Remove `T8Code` (to a different repo/extension/...)?

Inspired by @sloede talk at Trudi yesterday I wonder if there is any real reason why we currently bother dragging `T8Code` around. I vaguely remember @ranocha questioning back in the days what this mesh gives us, and what was conjectured back then was the possibility to have hybrid meshes which would rely on `DGMulti` (right, @jlchan ?) I am not sure if anybody is working actively on this, but I am almost sure nobody uses it at the moment. I am also raising this issue as a consequence of https://github.com/trixi-framework/Trixi.jl/pull/2847 where I struggle to get both `P4est` and `T8Code` right.