Remove the "Add a Git Repository" page

This page was written prior to the Apertis project using gitlab. The
content it contains is either replicated in the the contribution
documentation or describes defunct processes that would be unavailable to
the average reader of the website (due to no access to Phabricator) even if
they were still true.

Remove this document to avoid confusion.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>