Remove light client infrastructure from phase 0

[JustinDrake]

Remove light client infrastructure (active index roots and crosslink committee roots) from phase 0. Alternative infrastructure (namely, persistent committee roots) will be added in phase 1 for light clients (following an improved design). Benefits:

• Significantly less hashing for the beacon chain (persistent committee roots are relatively rarely updated)
• Code simplification (40 lines removed)
• Conceptual cleanup with all light client infrastructure in phase 1

[hwwhww]

Looks good to me. 👍
Note that #1316 needs some updates once this PR has been merged.

[vbuterin]

Does it? AFAIK the light client PR only uses the added logic in #1323.

[hwwhww]

@vbuterin we will have to copy the definition of CompactCommittee to #1316 if #1329 gets merged first.

class CompactCommittee(Container):
    pubkeys: List[BLSPubkey, MAX_VALIDATORS_PER_COMMITTEE]
    compact_validators: List[uint64, MAX_VALIDATORS_PER_COMMITTEE]