Fix deletion of categories

Author: padix-key

src/biotite/structure/io/pdbx/bcif.py

@@ -511,7 +511,7 @@ def __setitem__(self, key, element):
 
     def __delitem__(self, key):
         try:
-            return super().__setitem__("_" + key)
+            return super().__delitem__("_" + key)
         except KeyError:
             raise KeyError(key)