Ethereum Light Client - Subtract missing signers instead of aggregating signers

[hussein-aitlahcen]

As described in https://eth2book.info/altair/part3/containers/dependencies/#synccommittee, subtracting the missing signers from the trusted committee is faster and cheaper than aggregating the signers. In practice, more than 90% of the committee are signing, which is a potential 90% perf/size of update gain.