Remove the option to choose `yml`

[stijns96]

With a bit of digging, I found out that the preferred extension is yaml. yml is still there for historical use for devs that think that extensions should be a max of 3 chars.

For me it's quite unnecessary to have this option in Studio as both doing the same thing. Not sure how others think about this, but it looks like creating your file with yaml is already preferred since 2006.

Let me know :)