Replace image-url asset helper instances

Replace instances with CSS url() function. See rails/dartsass-rails#18.

Co-Authored-By: Martin Jones <[email protected]>

Author: jon-kirwan

app/assets/stylesheets/components/_subscribe.scss

@@ -10,12 +10,12 @@
   @include govuk-font($size: 19, $weight: bold);
 
   display: block;
-  background: image-url("calendars/icon-calendar.png") no-repeat scroll 0 0;
+  background: url("calendars/icon-calendar.png") no-repeat scroll 0 0;
   min-height: 2.5em;
   padding: 0 0 0 3em;
 
   @include govuk-device-pixel-ratio {
-    background-image: image-url("calendars/icon-calendar-2x.png");
+    background-image: url("calendars/icon-calendar-2x.png");
     background-size: 29px 24px;
   }
 }

app/assets/stylesheets/views/_calendars.scss

@@ -18,7 +18,7 @@
 }
 
 .app-bunting__triangles {
-  background-image: image-url("calendars/bunting-grey-string.svg");
+  background-image: url("calendars/bunting-grey-string.svg");
   background-color: transparent;
   background-repeat: repeat-x;
   background-position: center center;
@@ -38,7 +38,7 @@
 }
 
 .app-bunting__tinsel {
-  background-image: image-url("calendars/festive-lights-tinsel.svg");
+  background-image: url("calendars/festive-lights-tinsel.svg");
   background-color: transparent;
   background-repeat: repeat-x;
   background-position: center top;