Clear menu_router code out of function menu_local_tasks() when all actions and tabs are converted to plugins

follow-up to #2004334: Separate Tabs (MENU_LOCAL_TASK) from hook_menu()

function menu_local_tasks() should be reduced to interacting with the plugin managers and invoking a couple hooks.

+    // @todo remove all code using {menu_router} and anything using MENU_*
+    // constants when all local actions and local tasks are converted to
+    // plugins. The remaining code should just invoke those managers plus do the
+    // invocations of hook_menu_local_tasks() and hook_menu_local_tasks_alter().