Fix bug #625686 by removing the warning filter added by selftest.
* bzrlib/symbol_versioning.py: (suppress_deprecation_warnings, activate_deprecation_warnings): Always return a cleanup callable even if no filter is installed.
* bzrlib/versionedfile.py: Drive-by cleanup, the module is not needed.
* bzrlib/builtins.py: (cmd_selftest.run): Ensure we restore the right warning filters.