Remove previously deprecated main-wrapper and main-wrapper--l mixins

In ade03d4
we deprecated the `main-wrapper` and `main-wrapper--l` mixins because we didn't think they
were useful.

This commit removes those mixins. Note: the classes remain, but the mixins are being removed.

src/govuk/objects/_main-wrapper.scss

@@ -21,37 +21,22 @@
 //   </div>
 // </div>
 
-/// @deprecated Replace this mixin with more direct references to the [spacing
-/// mixins](https://design-system.service.gov.uk/styles/spacing/#spacing-on-custom-components).
-@mixin govuk-main-wrapper {
-  // In IE11 the `main` element can be used, but is not recognized  –
-  // meaning it's not defined in IE's default style sheet,
-  // so it uses CSS initial value, which is inline.
-  display: block;
-  padding-top: govuk-spacing(4);
-  padding-bottom: govuk-spacing(4);
-
-  @include govuk-media-query($from: tablet) {
-    // This spacing is manually adjusted to replicate the margin of
-    // govuk-heading-xl (50px) minus the spacing of back link and
-    // breadcrumbs (10px)
-    padding-top: govuk-spacing(7);
-    padding-bottom: govuk-spacing(7);
-  }
-}
-
-/// Use govuk-main-wrapper--l when you page does not have Breadcrumbs, phase
-/// banners or back links.
-///
-/// @deprecated Replace this mixin with more direct references to the [spacing
-/// mixins](https://design-system.service.gov.uk/styles/spacing/#spacing-on-custom-components).
-@mixin govuk-main-wrapper--l {
-  @include govuk-responsive-padding(8, "top");
-}
-
 @include govuk-exports("govuk/objects/main-wrapper") {
   .govuk-main-wrapper {
-    @include govuk-main-wrapper;
+    // In IE11 the `main` element can be used, but is not recognized  –
+    // meaning it's not defined in IE's default style sheet,
+    // so it uses CSS initial value, which is inline.
+    display: block;
+    padding-top: govuk-spacing(4);
+    padding-bottom: govuk-spacing(4);
+
+    @include govuk-media-query($from: tablet) {
+      // This spacing is manually adjusted to replicate the margin of
+      // govuk-heading-xl (50px) minus the spacing of back link and
+      // breadcrumbs (10px)
+      padding-top: govuk-spacing(7);
+      padding-bottom: govuk-spacing(7);
+    }
   }
 
   // Using the `.govuk-main-wrapper--auto-spacing` modifier should apply the
@@ -63,6 +48,6 @@
   // `govuk-main-wrapper--l` modifier instead.
   .govuk-main-wrapper--auto-spacing:first-child,
   .govuk-main-wrapper--l {
-    @include govuk-main-wrapper--l;
+    @include govuk-responsive-padding(8, "top");
   }
 }