Remove `git_repository_gc` from the headers

include/git2/repository.h

@@ -155,21 +155,17 @@ GIT_EXTERN(int) git_repository_index(git_index **index, git_repository *repo);
 /**
  * Free a previously allocated repository
  *
+ * Note that after a repository is free'd, all the objects it has spawned
+ * will still exist until they are manually closed by the user
+ * with `git_object_close`, but accessing any of the attributes of
+ * an object without a backing repository will result in undefined
+ * behavior
+ *
  * @param repo repository handle to close. If NULL nothing occurs.
  */
 GIT_EXTERN(void) git_repository_free(git_repository *repo);
 
 /**
- * Force a garbage collector pass on the repository
- *
- * This will force-free any cached objects that have been
- * previously marked by the user as closed (`git_object_close`).
- *
- * @param repo repository handle to collect. If NULL nothing occurs.
- */
-GIT_EXTERN(int) git_repository_gc(git_repository *repo);
-
-/**
  * Creates a new Git repository in the given folder.
  *
  * TODO: