[mle] remove AppendDiscovery declaration (#2273)
[mle] remove AppendDiscovery declaration (#2273)


erja-gp/openthread,openthread/openthread,LeZhang2016/openthread,erja-gp/openthread,chshu/openthread,fbsder/openthread,abtink/openthread,abtink/openthread,xiaom-GitHub/openthread,librasungirl/openthread,turon/openthread,jwhui/openthread,bukepo/openthread,gandreello/openthread,erja-gp/openthread,gandreello/openthread,georgecpr/openthread,lanyuwen/openthread,LeZhang2016/openthread,xiaom-GitHub/openthread,fbsder/openthread,lanyuwen/openthread,georgecpr/openthread,lanyuwen/openthread,pvanhorn/openthread,erja-gp/openthread,bukepo/openthread,chshu/openthread,fbsder/openthread,erja-gp/openthread,bukepo/openthread,pvanhorn/openthread,fbsder/openthread,jwhui/openthread,srickardti/openthread,erja-gp/openthread,chshu/openthread,pvanhorn/openthread,mszczodrak/openthread,srickardti/openthread,abtink/openthread,jwhui/openthread,pvanhorn/openthread,georgecpr/openthread,openthread/openthread,srickardti/openthread,bukepo/openthread,mszczodrak/openthread,mszczodrak/openthread,LeZhang2016/openthread,gandreello/openthread,srickardti/openthread,librasungirl/openthread,xiaom-GitHub/openthread,openthread/openthread,turon/openthread,LeZhang2016/openthread,abtink/openthread,georgecpr/openthread,georgecpr/openthread,chshu/openthread,lanyuwen/openthread,turon/openthread,turon/openthread,lanyuwen/openthread,chshu/openthread,fbsder/openthread,pvanhorn/openthread,pvanhorn/openthread,xiaom-GitHub/openthread,LeZhang2016/openthread,gandreello/openthread,LeZhang2016/openthread,mszczodrak/openthread,lanyuwen/openthread,jwhui/openthread,librasungirl/openthread,xiaom-GitHub/openthread,mszczodrak/openthread,gandreello/openthread,gandreello/openthread,chshu/openthread,fbsder/openthread,mszczodrak/openthread,openthread/openthread,xiaom-GitHub/openthread,librasungirl/openthread,georgecpr/openthread