Merged usr as default profile

[JeroenMathon]

Due to split usr being deprecated i reccomend switching the default profile to the merged-usr one

[amette]

Upstream made the merged-usr the default. And split-usr was dropped. So all that's needed is removing the split-usr support from gentoo-install.

[oddlama]

Correct me if I'm wrong but I don't think there is anything we need to change since there is nothing specifically selecting split-usr. If merged-usr is the new default and the profile names didn't change then I think there's nothing left to do :)