NO BUG - Remove deleted python/codegen from Sphinx docs

The python/codegen directory was removed in 9cbc28021b7b (bug 1108294).
However, the Sphinx reference lingered. Kill the dangling reference.

python/moz.build

@@ -5,7 +5,6 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 SPHINX_PYTHON_PACKAGE_DIRS += [
-    'codegen',
     'mach',
     'mozbuild/mozbuild',
     'mozbuild/mozpack',