lib/pull: Drop duplicate/different error for GPG but no detached meta
lib/pull: Drop duplicate/different error for GPG but no detached meta

Prep for a later patch to do GPG verification before writing commit objects;
`_ostree_repo_gpg_verify_with_metadata()` already handles this, and so dropping
this gives us consistent error messages.

Closes: #1269
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree