Get rid of the `GetShare` pattern that loops over share map

[GalRogozinski]

Description

We have places in code where we assume that the first share has common properties with all other committee shares.
So we extract the first share to get the data.
Even though correct, this is hacky, and we shouldn't do this.

e.g.

ssv-spec/ssv/runner_signatures.go

Lines 49 to 58 in b843e38

	// Get committee (unique for runner)
	var shareSample *types.Share
	for _, share := range b.Share {
	shareSample = share
	break
	}
	if shareSample == nil {
	return errors.New("can not get committee because there is no share in runner")
	}
	committee := shareSample.Committee

[GalRogozinski]

#464 is one way of doing it