Remove dummy threading modules (#13771)

Removed in Python 3.8, part of #12112

Author: srittau

stdlib/VERSIONS

@@ -36,8 +36,6 @@ _curses: 3.0-
 _curses_panel: 3.0-
 _dbm: 3.0-
 _decimal: 3.3-
-_dummy_thread: 3.0-3.8
-_dummy_threading: 3.0-3.8
 _frozen_importlib: 3.0-
 _frozen_importlib_external: 3.5-
 _gdbm: 3.0-
@@ -140,7 +138,6 @@ distutils: 3.0-3.11
 distutils.command.bdist_msi: 3.0-3.10
 distutils.command.bdist_wininst: 3.0-3.9
 doctest: 3.0-
-dummy_threading: 3.0-3.8
 email: 3.0-
 encodings: 3.0-
 encodings.cp1125: 3.4-