remove extra upgrade guides

No reason to provide upgrade guides between old versions when newer guides to newer versions exists.

* 5.1-to-5.2.md and 5.1-to-5.4.md can be removed because 5.1-to-6.0.md exists.
* 6.0-to-6.1.md can be removed because 6.0-to-7.2.md exists.