Remove duplicate font declarations for point 14

We can remove the underscore in `_14` → `14` so the deprecated font declaration map continue to work without duplication

packages/govuk-frontend/src/govuk/helpers/_typography.scss

@@ -160,6 +160,11 @@
   // deprecated sizes used internally, for example `govuk-font($size: "_14")`
   $size-internal-use-only: str-slice(#{$size}, 1, 1) == "_";
 
+  // Remove underscore from font sizes flagged for internal use
+  @if $size-internal-use-only {
+    $size: str-slice(#{$size}, 2);
+  }
+
   // Check for a font map exactly matching the given size
   $font-map: map-get($govuk-typography-scale, $size);
 

packages/govuk-frontend/src/govuk/settings/_typography-responsive.scss

@@ -157,19 +157,5 @@ $govuk-typography-scale: (
       message: "14 on the type scale is deprecated and will be removed as " +
         "a possible option in the next major version."
     )
-  ),
-  _14: (
-    null: (
-      font-size: 12px,
-      line-height: 15px
-    ),
-    tablet: (
-      font-size: 14px,
-      line-height: 20px
-    ),
-    print: (
-      font-size: 12pt,
-      line-height: 1.2
-    )
   )
 ) !default;