Remove all elasticsearch specific language from core

[jonhealy1]

Removing es-specific language from core.py and the core module will make it easier for other databases to plug into the infrastructure.

Example: In the get_all_collections function '_source' is referenced:

return Collections(
collections=[
self.collection_serializer.db_to_stac(c["_source"], base_url=base_url)
for c in hits
],
links=links,
)

This should be removed from the response that comes via database_logic.py in both of the backends.