Use govuk-frontend spacing

Author: andysellick

app/assets/stylesheets/components/_error-message.scss

@@ -2,7 +2,7 @@
 
 .app-c-error-message {
   display: block;
-  margin-bottom: $app-spacing-scale-3;
+  margin-bottom: govuk-spacing(3);
   clear: both;
   color: $govuk-error-colour;
   @include govuk-font(19, $weight: bold);

app/assets/stylesheets/components/helpers/_variables.scss

@@ -1,15 +1,3 @@
-// Forked from GOV.UK Frontend, namespace changed to ensure no conflicts.
-
-$app-spacing-scale-0: 0;
-$app-spacing-scale-1: 5px;
-$app-spacing-scale-2: 10px;
-$app-spacing-scale-3: 15px;
-$app-spacing-scale-4: 20px;
-$app-spacing-scale-5: 30px;
-$app-spacing-scale-6: 40px;
-$app-spacing-scale-7: 50px;
-$app-spacing-scale-8: 60px;
-
 $app-text-colour: $govuk-text-colour;
 $app-secondary-text-colour: govuk-colour("dark-grey", $legacy: "grey-1");