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.
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