[slaac] remove otIp6SlaacUpdate leftover from the ip6.h header (#3612)
[slaac] remove otIp6SlaacUpdate leftover from the ip6.h header (#3612)


bukepo/openthread,chshu/openthread,srickardti/openthread,turon/openthread,srickardti/openthread,bukepo/openthread,lanyuwen/openthread,turon/openthread,lanyuwen/openthread,jwhui/openthread,librasungirl/openthread,abtink/openthread,jwhui/openthread,chshu/openthread,openthread/openthread,librasungirl/openthread,openthread/openthread,bukepo/openthread,abtink/openthread,chshu/openthread,srickardti/openthread,librasungirl/openthread,openthread/openthread,lanyuwen/openthread,openthread/openthread,abtink/openthread,bukepo/openthread,srickardti/openthread,lanyuwen/openthread,chshu/openthread,lanyuwen/openthread,jwhui/openthread,jwhui/openthread,turon/openthread,librasungirl/openthread,turon/openthread,abtink/openthread,lanyuwen/openthread,chshu/openthread,chshu/openthread