Remove deprecated SCSS variables

- $govuk-new-link-styles is now set to true by default
- The other SCSS variables are deprecated from v5 of govuk-frontend

app/assets/stylesheets/application.scss

@@ -1,8 +1,4 @@
 // The main application stylesheet
-$govuk-compatibility-govuktemplate: false;
-$govuk-typography-use-rem: false;
-$govuk-use-legacy-palette: false;
-$govuk-new-link-styles: true;
 
 // This flag stops the font from being included in this application's
 // stylesheet - the font is being served by Static across all of GOV.UK, so is

app/assets/stylesheets/components/_contents-list-with-body.scss

@@ -23,7 +23,7 @@
 }
 
 .app-c-contents-list-with-body__link-wrapper.sticky-element--stuck-to-window {
-  background-color: govuk-colour("light-grey", $legacy: "grey-3");
+  background-color: govuk-colour("light-grey");
   bottom: -1px; // 'Fix' for anomalous 1px margin which sporadically appears below this element.
   left: 0;
   margin: 0;

app/assets/stylesheets/helpers/_parts.scss

@@ -9,7 +9,7 @@
       margin-bottom: 0;
     }
 
-    border-bottom: 1px solid govuk-colour("mid-grey", $legacy: "grey-2");
+    border-bottom: 1px solid govuk-colour("mid-grey");
   }
 
   .part-navigation {

app/assets/stylesheets/views/_manual.scss

@@ -61,7 +61,7 @@
   list-style: none;
 
   &:hover {
-    background-color: govuk-colour("light-grey", $legacy: "grey-4");
+    background-color: govuk-colour("light-grey");
   }
 
   &:last-child {