Remove webkit-clutter-layout test definition

This test is being deprecated and will be replaced by a new
webkit2gtk layout test.

Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D4149