Remove `.gitmodules`
Remove `.gitmodules`

We don’t use submodules any more. Having this file stick around is
causing problems in Atom, detailed here:
https://github.com/libgit2/libgit2/issues/3414

gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin