Bellatrix light client data does not contain the EL block hash, so we
had to follow blocks gossip to learn the EL `block_hash` of such blocks.
Now that Bellatrix is obsolete, we can simplify EL syncing logic under
light client scenarios. Bellatrix light client data can still be used
to advance the light client sync itself, but will no longer result in
`engine_forkchoiceUpdated` calls until the sync reaches Capella. This
also frees up some memory as we no longer have to retain blocks.
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