keep mobile link underline the same colour as the text

Author: chris-lawton

tbx/static_src/sass/components/navigation/_primary-nav-mobile.scss

@@ -64,7 +64,8 @@ Styles for the top level of the navigation at mobile
         &--no-children {
             @include link-styles(
                 $color: var(--color--heading),
-                $interaction-color: var(--color--heading)
+                $interaction-color: var(--color--heading),
+                $underline-color: var(--color--heading)
             );
         }
     }

tbx/static_src/sass/components/navigation/_sub-nav-mobile.scss

@@ -59,7 +59,8 @@ Here we refer to 'level 1 children' as 'level 2' and 'level 2 children' as 'leve
         &--no-children {
             @include link-styles(
                 $color: var(--color--heading),
-                $interaction-color: var(--color--heading)
+                $interaction-color: var(--color--heading),
+                $underline-color: var(--color--heading)
             );
             @include font-size('body-two');
             padding: $spacer-mini-plus $spacer-small;
@@ -79,7 +80,8 @@ Here we refer to 'level 1 children' as 'level 2' and 'level 2 children' as 'leve
         &--parent {
             @include link-styles(
                 $color: var(--color--heading),
-                $interaction-color: var(--color--heading)
+                $interaction-color: var(--color--heading),
+                $underline-color: var(--color--heading)
             );
         }
     }