sparse: drop the useless wrapping of `dirstate.set_untracked`

The code of the wrapper only act in the case where the file is not tracked, so this has not effect for `untracked`. In addition the message explicitly mention `add` of a file and no test are breaking if we drop this.

So we drop this for simplicity and cleanup

hgext/sparse.py

@@ -215,7 +215,6 b' def _setupdirstate(ui):'
     # Prevent adding files that are outside the sparse checkout
     editfuncs = [
         b'set_tracked',
-        b'set_untracked',
         b'copy',
     ]
     hint = _(