Removes start publication from checkpoint

[LeoPatOZ]

As we have decided not to allow for parallel proving (i.e. proving publications not starting from the latest proven), we no longer should / need to accept 'start' checkpoint when proving a transition - it will always be the latest proven checkpoint.

Slightly more gas efficient

Im not sure if we need to check L238 in the prover manager

[github-actions]

Changes to gas cost

Generated at commit: c299f04050b2dd772c2670a832acd35d323afc18, compared to commit: 6d08743d2312340033442e3745582597c7041ff9

🧾 Summary (10% most significant diffs)

Contract	Method	Avg (+/-)	%
CheckpointTracker	proveTransition	-860 ✅	-1.17%
ProverManager	prove	-766 ✅	-0.76%

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Full diff report 👇

Contract	Deployment Cost (+/-)	Method	Min (+/-)	%	Avg (+/-)	%	Median (+/-)	%	Max (+/-)	%	# Calls (+/-)
CheckpointTracker	809,718 (-100,114)	proveTransition	72,814 (-860)	-1.17%	72,814 (-860)	-1.17%	72,814 (-860)	-1.17%	72,814 (-860)	-1.17%	1 (0)
ProverManager	2,850,936 (-3,494)	prove	85,943 (-766)	-0.88%	99,606 (-766)	-0.76%	101,556 (-766)	-0.75%	111,314 (-766)	-0.68%	6 (0)