fix(gui): reflect folder password visibility by changing button icon (#9857)

Make the "toggle password visibility" button on encrypted
folders change its icon from `fa-eye` to `fa-eye-slash` while the
password is visible.

gui/default/syncthing/core/syncthingController.js

@@ -3703,7 +3703,6 @@ angular.module('syncthing.core')
                 untrusted: '=',
             },
             link: function (scope, elem, attrs) {
-                var plain = false;
                 scope.togglePasswordVisibility = function() {
                     scope.plain = !scope.plain;
                 };