Remove use-gtk feature

[cmyr]

The intent of this feature was to allow selectively using gtk on mac, which was specifically so that I could test the gtk backend without needing to boot linux.

Ultimately it's way easier for me to just run a VM than to maintain this feature, and it's an annoying burden, maintenance wise.

Removing this feature will involve\

• removing it from the Cargo.toml (druid & druid-shell)
• removing it anywhere it is used; we should use the gtk backend by default on linux, only swapping to the x11 backend when it is specified by feature;
• and updating CI to no longer use this flag anywhere, if it is currently.

This should wait until #599 lands, otherwise it will be a painful merge.