Remove 'base' import from files in overrides layer

Importing files from this layer without first importing the 'base' file was deprecated in #1807. Anyone who was relying on the import should already be seeing warnings. If this change affects them, they'll just need to ensure they're importing the base layer first.

This is part of a change to the way that the base layers (settings, tools and helpers) are imported within different parts of GOV.UK Frontend’s Sass, designed to reduce the time it takes to compile the Sass to CSS.

Remove the tests, which were only checking that the deprecation warning was emitted as expected.

src/govuk/overrides/_display.scss

@@ -1,9 +1,3 @@
-@if not mixin-exists("govuk-exports") {
-  @warn "Importing items from the overrides layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
-}
-
-@import "../base";
-
 // stylelint-disable declaration-no-important
 @include govuk-exports("govuk/overrides/display") {
   .govuk-\!-display-inline {

src/govuk/overrides/_spacing.scss

@@ -1,9 +1,3 @@
-@if not mixin-exists("govuk-exports") {
-  @warn "Importing items from the overrides layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
-}
-
-@import "../base";
-
 ////
 /// @group overrides
 ////

src/govuk/overrides/_text-align.scss

@@ -1,9 +1,3 @@
-@if not mixin-exists("govuk-exports") {
-  @warn "Importing items from the overrides layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
-}
-
-@import "../base";
-
 // stylelint-disable declaration-no-important
 @include govuk-exports("govuk/overrides/text-align") {
   .govuk-\!-text-align-left {

src/govuk/overrides/_typography.scss

@@ -1,9 +1,3 @@
-@if not mixin-exists("govuk-exports") {
-  @warn "Importing items from the overrides layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
-}
-
-@import "../base";
-
 @include govuk-exports("govuk/overrides/typography") {
   // Font size and line height
 

src/govuk/overrides/_width.scss

@@ -1,9 +1,3 @@
-@if not mixin-exists("govuk-exports") {
-  @warn "Importing items from the overrides layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
-}
-
-@import "../base";
-
 // stylelint-disable declaration-no-important
 @include govuk-exports("govuk/overrides/width") {
   .govuk-\!-width-full {