lib/core: Add #defines for ref/collection binding

These were previously private, but since we expect people to use them, let's add
`#define`s like we did for some of the other commit metadata.

Closes: #1028
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -56,10 +56,6 @@ G_BEGIN_DECLS
  * in a summary file. */
 #define OSTREE_COMMIT_TIMESTAMP "ostree.commit.timestamp"
 
-/* Well-known keys for the commit metadata */
-#define OSTREE_REF_BINDING "ostree.ref-binding"
-#define OSTREE_COLLECTION_BINDING "ostree.collection-binding"
-
 typedef enum {
   OSTREE_REPO_TEST_ERROR_PRE_COMMIT = (1 << 0)
 } OstreeRepoTestErrorFlags;