sysroot: Add API to clean up transient keys in origin files

The `origin/unlocked` and `origin/override-commit` keys are examples of state
that's really transient; we don't want to maintain them across upgrades. Right
now there are bits for this in both `ostree admin upgrade` as well as in
rpm-ostree.

This new API will slightly clean up both cases, but it's really prep for adding
a concept of deployment "pinning" that will live in the new
`libostree-transient` group.

Closes: #1464
Approved by: jlebon

apidoc/ostree-sections.txt

@@ -173,6 +173,7 @@ ostree_deployment_set_index
 ostree_deployment_set_bootserial
 ostree_deployment_set_bootconfig
 ostree_deployment_set_origin
+ostree_deployment_origin_remove_transient_state
 ostree_deployment_clone
 ostree_deployment_unlocked_state_to_string
 <SUBSECTION Standard>