[MDX] Remove `frontmatterOptions` (#4204)

* feat: remove frontmatterOptions config

* test: remove custom frontmatter suite

* deps: remove remark-mdx-frontmatter

* docs: remove `frontmatterOptions` config

* chore: changeset

3 files changed, 0 insertions, 32 deletions

diff --git a/packages/integrations/mdx/test/fixtures/mdx-custom-frontmatter-name/src/pages/glob.json.js b/packages/integrations/mdx/test/fixtures/mdx-custom-frontmatter-name/src/pages/glob.json.js
deleted file mode 100644
index 2f8155ada..000000000
--- a/packages/integrations/mdx/test/fixtures/mdx-custom-frontmatter-name/src/pages/glob.json.js
+++ /dev/null
@@ -1,9 +0,0 @@
-export async function get() {
-	const mdxPages = await import.meta.glob('./*.mdx', { eager: true });
-
-	return {
-		body: JSON.stringify({
-			titles: Object.values(mdxPages ?? {}).map(v => v?.customFrontmatter?.title),
-		})
-	}
-}
diff --git a/packages/integrations/mdx/test/fixtures/mdx-custom-frontmatter-name/src/pages/index.mdx b/packages/integrations/mdx/test/fixtures/mdx-custom-frontmatter-name/src/pages/index.mdx
deleted file mode 100644
index e3c789149..000000000
--- a/packages/integrations/mdx/test/fixtures/mdx-custom-frontmatter-name/src/pages/index.mdx
+++ /dev/null
@@ -1,6 +0,0 @@
----
-title: 'Using YAML frontmatter'
-illThrowIfIDontExist: "Oh no, that's scary!"
----
-
-# {customFrontmatter.illThrowIfIDontExist}
diff --git a/packages/integrations/mdx/test/mdx-frontmatter.test.js b/packages/integrations/mdx/test/mdx-frontmatter.test.js
index 5f0316986..e69919bd5 100644
--- a/packages/integrations/mdx/test/mdx-frontmatter.test.js
+++ b/packages/integrations/mdx/test/mdx-frontmatter.test.js
@@ -56,21 +56,4 @@ describe('MDX frontmatter', () => {
 		expect(headingSlugs).to.contain('section-1');
 		expect(headingSlugs).to.contain('section-2');
 	});
-
-	it('extracts frontmatter to "customFrontmatter" export when configured', async () => {
-		const customFixture = await loadFixture({
-			root: new URL('./fixtures/mdx-custom-frontmatter-name/', import.meta.url),
-			integrations: [
-				mdx({
-					frontmatterOptions: {
-						name: 'customFrontmatter',
-					},
-				}),
-			],
-		});
-		await customFixture.build();
-
-		const { titles } = JSON.parse(await customFixture.readFile('/glob.json'));
-		expect(titles).to.include('Using YAML frontmatter');
-	});
 });