[mle] remove redundant `RemoveChildren` when router id changes (#4281)
[mle] remove redundant `RemoveChildren` when router id changes (#4281)


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