Remove checkpoint timestamp clamping in CheckpointBuilder (#27011)

## Description

Consensus commit timestamps are now guaranteed monotonic even across
epoch boundaries, so the CheckpointBuilder no longer needs to clamp a
decreasing checkpoint timestamp to the previous checkpoint's. A decrease
now signals a real invariant violation, so it is reported via
`debug_fatal!` instead of being silently corrected.

## Test plan

Covered by existing checkpoint-building tests in `sui-core`; the removed
branch is now unreachable under the monotonicity guarantee.

## Release notes

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