cubicweb #2719115 naive cache on rset.get_entity prevent garbage collection [in-progress]

In some unclear circumstance the @cached on ResultSet.get_entity prevert the result set to be garbage collected.

This cache should not be necessary as the unlying session already hold this kind of cache.

Remove this cache may impact web only config

done in<not specified>
load left0.200
closed by<not specified>
patch[rset] drop the cache over .get_entity [rejected]