Don't require PostGIS unless using it as a search backend

There is absolutely no reason to require PostGIS if you are using one of the Solr backends. It's just required for historical reasons as it was the only backend supported initially.

The PostGIS search backend is not recommended anyway and the PostGIS install adds complexity and causes issues like ckan/ckan#3929 or #182.

Start by locating where PostGIS is declared as a required dependency and where the Solr and PostGIS search backends are selected. Confirm that installations using Solr do not require PostGIS, while PostGIS remains available when that backend is chosen; update the relevant checks and tests if present.

Written by the indexing model from the issue text.