[Gorm] Remove unused Gorm documents array

[Graham Lee]

Hi folks,

I think when Gorm document management was switched to NSDocumentController, some methods weren't updated, so there's a documents collection that's never added to but is sometimes used. I've switched references to the shared document controller's collection in this PR: https://github.com/gnustep/apps-gorm/pull/2

or an old-fashioned patch: https://patch-diff.githubusercontent.com/raw/gnustep/apps-gorm/pull/2.patch

Thanks,
Graham.