chore(merkle): remove last walker key from checkpoint

[rkrasiuk]

Description

Ref #5048

Remove last_walker_key from merkle checkpoint. This PR targets breaking-changes branch since it's not backwards compatible.

[codecov]

Codecov Report

Merging #5069 (b970252) into breaking-changes (4b1a0ce) will decrease coverage by 0.06%.
Report is 2 commits behind head on breaking-changes.
The diff coverage is 50.00%.

Files	Coverage Δ
crates/stages/src/stages/merkle.rs	82.92% <ø> (+0.40%)	⬆️
crates/primitives/src/stage/checkpoints.rs	75.88% <50.00%> (-5.98%)	⬇️

... and 7 files with indirect coverage changes

Flag	Coverage Δ
integration-tests	17.02% <0.00%> (+0.01%)	⬆️
unit-tests	62.35% <50.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
reth binary	30.85% <ø> (ø)
blockchain tree	80.82% <ø> (ø)
pipeline	88.41% <ø> (+0.02%)	⬆️
storage (db)	74.41% <ø> (+<0.01%)	⬆️
trie	94.96% <ø> (-0.04%)	⬇️
txpool	55.33% <ø> (-0.45%)	⬇️
networking	78.19% <ø> (ø)
rpc	58.22% <ø> (-0.01%)	⬇️
consensus	63.01% <ø> (ø)
revm	28.69% <ø> (ø)
payload builder	7.95% <ø> (ø)
primitives	86.32% <50.00%> (-0.05%)	⬇️