BF: strip away any prefix before ":" while determining numeric dandiset identifier

We have it confusing, as the identifier in the client model is just the
numeric part. We should have called it differently from the metadata model identifier

Author: yarikoptic

dandi/dandiset.py

@@ -139,9 +139,9 @@ def _get_identifier(metadata: dict) -> str | None:
             id_ = str(id_.get("value", ""))
         elif id_ is not None:
             assert isinstance(id_, str)
-            if id_.startswith("DANDI:"):
-                # result of https://github.com/dandi/dandi-cli/pull/348 which
-                id_ = id_[len("DANDI:") :]
+            # result of https://github.com/dandi/dandi-cli/pull/348 which ???
+            # TODO: RF to avoid this evil!!!
+            id_ = id_.split(":")[-1]
 
         assert id_ is None or isinstance(id_, str)
         return id_