Cleanup: Removed md-file CSS class

What does this MR do?

So far, the .md-file class has always been used in combo with the simple .md. Hence, all the occurrences of this couple got updated to use .md only instead.

Screenshots

Providing a screenshot of the Snippet view as an example of one of the views where the refactoring took place

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content