extension: stop returning the results from the __parse() method

Now that parsing is a separate step, the results are no longer needed
while parsing.

src/hawkmoth/__init__.py

@@ -72,7 +72,7 @@ def __parse(self, filename):
         key = (filename, tuple(clang_args))
 
         if key in parsed_files:
-            return parsed_files[key]
+            return
 
         # Tell Sphinx about the dependency
         self.env.note_dependency(filename)
@@ -84,8 +84,6 @@ def __parse(self, filename):
 
         parsed_files[key] = docstrings
 
-        return docstrings
-
     def __parsed_files(self, filter_filenames=None, filter_clang_args=None):
         parsed_files = self.env.temp_data.get('hawkmoth_parsed_files', {})