RFC: autotests: Remove WAYLAND_ONLY tests
WAYLAND_ONLY will create two tests: one with Xwayland, the other without Xwayland. This is somewhat wasteful and it results in higher CI times.
On the other hand, Xwayland is started on demand. If a test doesn't need Xwayland, it won't start.
So let's remove WAYLAND_ONLY in order to lighten kwin on CI resources. If wayland only tests are needed, we can consider passing the operation mode to the WAYLANDTEST_MAIN helper, but there aren't such tests afaik.