Revert "Dialog: Add legacy z-index compatibility"

This reverts commit 72e288f.

Author: mirka

packages/ui/src/dialog/stories/index.story.tsx

@@ -175,17 +175,3 @@ export const AllSizes: Story = {
 		children: <SizePlaygroundContent />,
 	},
 };
-
-/**
- * Popovers in Gutenberg are managed with explicit z-index values, which can create
- * situations where a dialog renders below another popover, when you want it to be rendered above.
- *
- * The `--wp-ui-dialog-z-index` CSS variable controls the z-index of both the
- * backdrop and the popup. It can be overridden globally by setting the variable
- * on `:root` or `body`. (This story doesn't actually demonstrate the feature
- * because it requires a global CSS rule.)
- */
-export const WithCustomZIndex: Story = {
-	..._Default,
-	name: 'With Custom z-index',
-};

packages/ui/src/dialog/style.module.css

@@ -4,7 +4,6 @@
 	.backdrop {
 		position: fixed;
 		inset: 0;
-		z-index: var(--wp-ui-dialog-z-index, initial);
 		background-color: rgba(0, 0, 0, 0.35);
 
 		&[data-starting-style],
@@ -26,7 +25,6 @@
 		top: 50%;
 		/* stylelint-disable-next-line plugin/use-logical-properties-and-values -- Physical centering technique used with transform: translate(-50%, -50%) */
 		left: 50%;
-		z-index: var(--wp-ui-dialog-z-index, initial);
 		transform: translate(-50%, -50%);
 		box-sizing: border-box;
 		min-width: 320px;