compiler: Revert support for `None` as `TNone`

This was mistakenly included in fb2b634c4a, and broke the test
case verifying that using None as an ARTIQ type annotation in fact
generates an error message.

artiq/compiler/embedding.py

@@ -950,9 +950,6 @@ class Stitcher:
         return function_node
 
     def _extract_annot(self, function, annot, kind, call_loc, fn_kind):
-        if annot is None:
-            annot = builtins.TNone()
-
         if not isinstance(annot, types.Type):
             diag = diagnostic.Diagnostic("error",
                 "type annotation for {kind}, '{annot}', is not an ARTIQ type",