x11-toolkits/gtk30: optionalize at-spi2-atk dependency

AT-SPI bridge requires DBus and is limited to X11. As upstream plans
to rework accessibility only in Gtk4 revert to make the bridge optional.
And Gtk 3.26 isn't on the horizon yet.

Similar changes:
https://github.com/netbsd/pkgsrc/commit/1248d6517109
https://github.com/DragonFlyBSD/DeltaPorts/commit/6009bc6e5010

PR: 194460
Tested by: many
Approved by: maintainer timeout (5 years)