Attester: IBC sign moded

[randygrok]

// Only process if sign mode is IBC_ONLY and we have outbound IBC packets
//if params.SignMode == types.SignMode_SIGN_MODE_IBC_ONLY {
// return errors.New("IBC only sign mode not yet implemented")
//}

ref: https://github.com/evstack/ev-abci/pull/230/files/0484c62f18686888e2c7cb8bfdd5c0a9510ccecb#diff-6399337291dfbfe9342b749f29552fe29f3d9e3eaf83a1d55322958be2d5ae6fR20

[julienrbrt]

I do not think we need this at all