[chore] delete koko_analytics_last_aggregation_at on uninstall, document options explicitly not deleted

Author: dannyvankooten

uninstall.php

@@ -18,3 +18,7 @@
 delete_option("koko_analytics_use_custom_endpoint");
 delete_option("koko_analytics_realtime_pageview_count");
 delete_option('koko_analytics_jetpack_import_params');
+delete_option('koko_analytics_last_aggregation_at');
+
+// not removing koko_analytics_version because the database tables itself are not removed upon uninstall 
+// not removing koko_analytics_referrers_v2 because it also describes database state