Remove unsupported options for markdown

The ubuntu/focal repository provides an old version of the markdown
package which doesn’t support the jekyllData and taskLists options.
Since they are not used at the moment, they are commented out.
Reactivating them would require setting up an additional apt repository.

preamble.tex

@@ -28,11 +28,11 @@
     % ---
     % title: 'This is the title'
     % ---
-    jekyllData,
+    %jekyllData,
     pipeTables,
     % Usage: `: Caption`
     tableCaptions,
-    taskLists,
+    %taskLists,
     % Disable _emphasis_, only *emphasis* is allowed
     underscores=false
 ]{markdown}