Remove Documents support

In theory, online integration for documents is still desired. However,
during the GNOME 3.31.x / 3.32 development cycle, GNOME Documents was
removed from the set of core applications [1] due to neglect from
designers and developers. Therefore, there is no longer any
application to offer this feature.

[1] gnome-build-meta commit 3b267376306ef600
    releng commit 4fa9d6b174f97427
    https://gitlab.gnome.org/GNOME/gnome-build-meta/merge_requests/157

https://gitlab.gnome.org/GNOME/gnome-online-accounts/-/merge_requests/66