[cwconfig] Remove useless 'ui-cube' configuration option (closes #3728904)
The mechanism introduced could never actually be used as it brings dependency
loops to the cube graph. Just get rid of it.
This reverts commit 4032499c701e.
usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl