fix: remove deprecated dj.key from __all__

dj.key was removed in 2.0 but was still listed in __all__ without
being imported or defined.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko

src/datajoint/__init__.py

@@ -56,7 +56,6 @@
     "errors",
     "migrate",
     "DataJointError",
-    "key",
     "key_hash",
     "logger",
     "cli",