Delete variables when uninstall module.
Bug fixes will only go in the 6.x branch.
#2641906: Table advagg_bundler_selector_count missing after update to version 6.x-1.10#2853967: Error in advagg_bundler_get_css_selector_count - missing ctools css
Issue #2855388 by jurgenhaas: Use new IV for each request
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