Remove outdated includes replaced by SDCs for next major version

[ctorgalson]

@ctorgalson
@markconroy

When adding several of the SDCs in localgov_base, we retained the previously-used Twig include files (like e.g. the old prev-next.twig include) for backwards compatibility.

We probably should have removed them in 2.0, but we should definitely remove them in time for the 3.0 release.