Description

In #1639 the canonical_head_block_root field of PersistedBeaconChain was rendered obsolete by the use of fork choice to derive the head block on startup. We intended to remove it entirely when we did the breaking schema change for v0.3.0, but that PR (#1638) got closed and forgotten about ☹️

As a practice migration, I think we should remove the field entirely in a future release, i.e. automatically update the user's database from the old schema (with the block root) to the new (without) on startup.