[server] fix 'cnx' variable confusion in DBG_SQL exception case The rollback handling expects 'cnx' to be the cubicweb Connection, but the DBG_SQL block was replacing it with an sql connection, leading to lulz down the line. Also remove obsolete getattr (the sqlite wrapping is now done at the cnxset level, so cnx.cnxset.cnx should be the right thing already).

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