[test] drop CubicWebConfigurationWithLegacyCubesTC
This is used to test legacy cube import which is planned for removal
in cubicweb 3.27, so drop the code.
Cubes=====This chapter describes how to define your own cubes and reuse already available cubes...toctree:::maxdepth: 1 layout.rst cc-newcube.rst available-cubes.rst