Loader: Make _extract_depends_from_node not 'del' nonexistent fields

This is benign because a _yaml.ChainMap doesn't raise a
KeyError, but it's not required to be a ChainMap.

buildstream/_loader/loadelement.py

@@ -185,6 +185,6 @@ def _extract_depends_from_node(node, *, key=None):
         output_deps.append(dependency)
 
     # Now delete the field, we dont want it anymore
-    del node[key]
+    node.pop(key, None)
 
     return output_deps