Remove `printng` calls from WMS thumbnail generation

[bieganowski]

In case of Geoserver OGC backend, the WMS thumbnail generation is executed as a fallback, in case Geoserver's printng fails:

geonode/geonode/layers/utils.py

Line 1008 in 0a82a14

image = _prepare_thumbnail_body_from_opts(request_body)

With the new approach, this should be refactored not to use printng at all. Approach for non-geoserver backend should be executed for all requets; the approach following:

geonode/geonode/layers/utils.py

Line 1013 in 0a82a14

if image is None: