This calls a FCU to set the unsafe head of the L2 execution client
immediately after NewPayload is called on the execution client. This
means that the unsafe head in the node state matches the unsafe head
on the execution client.

This commit removes the FCU prior to the NewPayload call because
we assert that the unsafe block that we are inserting extends the
current unsafe head. As such, the L2 execution client is in a state
to accept the new block.