GabbleOLPCGadgetManager: don't redundantly check TargetHandle
GabbleOLPCGadgetManager: don't redundantly check TargetHandle

TpBaseConnection already checks that the arguments are internally
consistent, so it's sufficient to check TargetHandleType.

community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,community-ssu/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,community-ssu/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble