Eliminates course no longer accepting enrollments (#5863)

One of Udemy URL to the course is no longer accepting enrollments.

courses/free-courses-en.md

@@ -534,7 +534,6 @@
 * [Introduction To Linux](https://www.edx.org/course/introduction-to-linux/) - The Linux Foundation (edx)
 * [Linux Administration Tutorial Videos](https://www.youtube.com/playlist?list=PL9ooVrP1hQOH3SvcgkC4Qv2cyCebvs0Ik) - edureka!
 * [Linux Command Line](https://www.udemy.com/course/command-line/) - Adam Eubankas (Udemy)
-* [Linux Operating System Fundamentals](https://www.udemy.com/course/linux-academy-linux-operating-system-fundamentals/) - Linux Academy (Udemy)
 * [Linux Tutorials and Projects](https://www.udemy.com/course/linux-tutorials/) - Jason Cannon (Udemy)
 * [Red Hat Enterprise Linux Technical Overview](https://www.udemy.com/course/red-hat-enterprise-linux-technical-overview/) - Red Hat, Inc. (Udemy)
 * [The Linux Basics: Beginner to Sysadmin, Step by Step](https://www.youtube.com/playlist?list=PLtK75qxsQaMLZSo7KL-PmiRarU7hrpnwK)