Get rid of oxide bits in //ui/gl/init

We currently replace the ozone bits in //ui/gl/init with Oxide implementations. This originates from a time when Chromium/Ozone builds didn't support GLX, but that's not the case anymore.

I want to try to get Chromium's content shell working from an Oxide build in order to run the blink layout tests. The content shell will communicate directly with the windowing system rather than using Oxide, and so will require a full featured ozone platform. As part of this, I'm planning to enable the x11 ozone platform for Oxide builds, but in order to make this work, the ozone bits in //ui/gl/init need to be re-enabled.