Remove the GTK backend

[DJMcNab]

As discussed in office hours today and #glazier > Near-term Linux pushes

To clarify reasoning, we don't have any will to maintain this backend - the other Linux backends need more attention

[xStrom]

This may be the right thing to do, but we shouldn't rush this.

I think we should wait on this until we have a clearer consensus on the Linux strategy. There have been many attempts to reach such a consensus in the past, but they've fizzled out.

I will go through historic issues and chat logs and create a Linux strategy meta issue here in the Glazier repo. Then we can have additional discussion on top. The idea is to have a clear idea of what are the pros and cons of each Linux backend and what the ideal future scenario looks like.

[DJMcNab]

To track this in the git history:

• We currently don't believe that the GTK backend can work with wgpu/Vulkan, and that is currently the only supported drawing scheme
• See also Deciding the Linux strategy #106

Therefore, we are having to remove this backend.