Remove retired and security branches from active docs (#3882)

Author: ned-deily

Doc/tools/static/version_switch.js

@@ -5,8 +5,6 @@
     '3.7': 'dev (3.7)',
     '3.6': '3.6',
     '3.5': '3.5',
-    '3.4': '3.4',
-    '3.3': '3.3',
     '2.7': '2.7',
   };