Get rid of genesis/terminalTotalDifficultyPassed

[karalabe]

Since Geth v1.14 can only run in post-merge mode, we can just assume that all networks are like that and don't even bother with checking? Tho perhaps we can leave the flag in the genesis for a major version and then remove it? Needs a bit of thought.

[karalabe]

Just a small triage update. We kind of all agree to remove this field, but maybe wait 1-2-3 small releases just to have time to phase out pre-merge networks.

[karalabe]

Support for non merge networks is being dropped in 1.14 (#29169), so this issue should be "fixed" in light of that timing wise.

[s1na]

Consensus on triage seems to be that the issue has marinated and now this field can be removed.

When it is removed: geth will fail on a network that isn't in PoS mode.

[jwasinger]

This was done in #30609