refactoring: clear privilege managing triggers from exceptions

modify_priv, revoke_priv & on_replace_dd_priv triggers are
cleared from exceptions. A list of functions: priv_def_check,
priv_def_create_from_tuple, user_grant_priv, user_reload_privs,
rebuild_effective_grants, grant_revoke, role_check, role_grant,
role_revoke, priv_grant, access_check_ddl & txn_alter_trigger_new
were also refactored to achieve it. Their usages are updated.
user_find_xc is removed as far as it is not needed anymore.

Part of #4247