Rejig configuration deleting again

Had second thoughts and deletion should only remove data, not sub
hierarchies.  If the entire hierarchy needs clearing out, it'll need to
be done manually, or by splicing in an empty HierarchicalDict.

volatility/framework/interfaces/configuration.py

@@ -148,17 +148,7 @@ class HierarchicalDict(collections.abc.Mapping):
                 subdict = self._subdict[self._key_head(key)]
                 del subdict[self._key_tail(key)]
             else:
-                key_head = self._key_head(key)
-                present = False
-                if key_head in self._data:
-                    present = True
-                    del self._data[self._key_head(key)]
-                if key_head in self._subdict:
-                    present = True
-                    del self._subdict[key_head]
-                if not present:
-                    # Make a call to throw the right exception
-                    del self._data[key_head]
+                del self._subdict[self._key_head(key)]
         except KeyError:
             raise KeyError(key)