remove redundant frontmatter parsing

lib/mdx.js

@@ -56,13 +56,11 @@ export async function getFileBySlug(type, slug) {
 
   let toc = []
 
-  // Parsing frontmatter here to pass it in as options to rehype plugin
-  const { data: frontmatter } = matter(source)
-  const { code } = await bundleMDX({
+  const { code, frontmatter } = await bundleMDX({
     source,
     // mdx imports can be automatically source from the components directory
     cwd: path.join(root, 'components'),
-    xdmOptions(options) {
+    xdmOptions(options, frontmatter) {
       // this is the recommended way to add custom remark/rehype plugins:
       // The syntax might look weird, but it protects you in case we add/remove
       // plugins in the future.