[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...

Remove usage of cnx.ensure_cnx_set context manager It has been deprecated since 3.21, no reason to keep using it internally.

[hooks/test/synschema] remove an unneeded skipTest. Related to #5557633.