[native] Use cnx.system_sql instead of doexec method in syntax_tree_search
The latter handles connection issues and tries to reconnect. Method
syntax_tree_search appears to be called from quite a few place where it seems
useful to be resilient to connection issues (authentication for instance).
This reconnection logic appears to have disappeared at some point when working
on https://www.cubicweb.org/2919309. Could not find the exact reason though...