t7112: remove mention of KNOWN_FAILURE_SUBMODULE_RECURSIVE_NESTED

The known failure mode KNOWN_FAILURE_SUBMODULE_RECURSIVE_NESTED was removed from lib-submodule-update.sh in 218c883783 (submodule: properly recurse for read-tree and checkout, 2017-05-02) but at that time this change was not ported over to topic sb/reset-recurse-submodules, such that when this topic was merged in 5f074ca7e8 (Merge branch 'sb/reset-recurse-submodules', 2017-05-29), t7112-reset-submodules.sh kept a mention of this removed failure mode. Remove it now, as it does not mean anything anymore. Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>