Update MissedAttestation dashboard and delete BlockDelays dashboard

[chong-he]

Fix the formula in MissedAttestation dashboard

For BlockDelays, I see that the metrics in the dashboard:

beacon_block_observed_slot_start_delay_time_sum
beacon_block_observed_slot_start_delay_time_count
beacon_block_observed_slot_start_delay_time_bucket
beacon_block_imported_observed_delay_time_sum
beacon_block_imported_observed_delay_time_count
beacon_block_imported_observed_delay_time_bucket
beacon_block_head_imported_delay_time_sum
beacon_block_head_imported_delay_time_count
beacon_block_head_imported_delay_time_bucket

are no longer available since the update in Lighthouse 5.2.0 (Reference: sigp/lighthouse#5510). As such, I propose to delete the BlockDelays dashboard. Users should use the updated MissedAttestation dashboard which provides finer details about block delays to replace the BlockDelays dashboard