remote-modify: Remove obsolete code

Now that flatpak_resolve_duplicate_remotes checks if a remote exists, we
don't need to use ostree_repo_remote_get_url to do so. This commit
doesn't change the behavior for remotes with empty URLs (which means
they're disabled), because the empty string is an allowed value as far
as OSTree is concerned.

Closes: #1247
Approved by: alexlarsson
This commit is contained in:
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f