datastore: restrict datastores list_images method scope to module

Drop the pub scope for `DataStore`s `list_images` method.

This method is only used to generate a list of index files found in
the datastore for iteration during garbage collection. There are no
other call sites and this is intended to only be used within the
module itself. Allows to be more flexible for future method signature
adaptions.

No functional changes.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>

pbs-datastore/src/datastore.rs

@@ -970,7 +970,7 @@ impl DataStore {
         ListGroups::new(Arc::clone(self), ns)?.collect()
     }
 
-    pub fn list_images(&self) -> Result<Vec<PathBuf>, Error> {
+    fn list_images(&self) -> Result<Vec<PathBuf>, Error> {
         let base = self.base_path();
 
         let mut list = vec![];