GSM: remove slot to wall-clock time translations

[amesgen]

A follow-up enabled by this PR (as the current chain now contains the slot times of its headers) would be to get rid of the time translations in the GSM:

ouroboros-consensus/ouroboros-consensus-diffusion/src/ouroboros-consensus-diffusion/Ouroboros/Consensus/Node/GSM.hs

Line 470 in a020b7b

let qry = Qry.slotToWallclock slot

Originally posted by @amesgen in #1288 (review)