Remove file cubicweb-slickgrid.spec

- It seemed now useless.
- It semmed to be for 1.1 and Python.
3 jobs for topic/default/no-spec in 31 seconds (queued for 18 seconds)
Name Stage Failure
pytest Tests
creating /usr/local/lib/python3.9/site-packages/rdflib_jsonld-0.5.0-py3.9.egg
Extracting rdflib_jsonld-0.5.0-py3.9.egg to /usr/local/lib/python3.9/site-packages
rdflib-jsonld 0.5.0 is already the active version in easy-install.pth

Installed /usr/local/lib/python3.9/site-packages/rdflib_jsonld-0.5.0-py3.9.egg
error: The 'rdflib' distribution was not found and is required by cubicweb
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: command terminated with exit code 1