[Routes] Review potential relicts from action suffix removals

[Guite]

• src/system/RoutesModule/Helper/SanitizeHelper.php: sanitizeAction method
• main controller - selecting controller/action for the main page in "general settings > start page"
• Routes module - creating custom routes - update code needed?
• Content module - selecting controller/action for controller elements - update code needed?

[craigh]

you created a select method for the start page selector which seems like it could be useful in the routes module at least if not elsewhere.

[craigh]

refs #4376