[meshcop] remove redundant message copying (#4254)
[meshcop] remove redundant message copying (#4254)

This commit removes redundant message copying when a joiner router is
handling a RLY_TX.ntf message.
lanyuwen/openthread,srickardti/openthread,lanyuwen/openthread,jwhui/openthread,lanyuwen/openthread,bukepo/openthread,jwhui/openthread,abtink/openthread,jwhui/openthread,librasungirl/openthread,openthread/openthread,abtink/openthread,abtink/openthread,bukepo/openthread,srickardti/openthread,srickardti/openthread,srickardti/openthread,librasungirl/openthread,lanyuwen/openthread,chshu/openthread,librasungirl/openthread,openthread/openthread,chshu/openthread,lanyuwen/openthread,chshu/openthread,librasungirl/openthread,abtink/openthread,bukepo/openthread,lanyuwen/openthread,chshu/openthread,chshu/openthread,openthread/openthread,bukepo/openthread,openthread/openthread,jwhui/openthread,chshu/openthread