[core] remove checkpoint executor unit testss (#20624)

## Description 

Unit testing checkpoint executor does not provide sufficient signal
compared to end to end simtests to warrant their continued existence. As
the system has evolved, the assumptions in these tests have all been
broken at some point or another, causing them to fail or become flaky.
Removing.

## Test plan 

👀 

---

## Release notes

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crates/sui-core/src/checkpoints/checkpoint_executor/mod.rs

@@ -70,9 +70,6 @@ use crate::{
 mod data_ingestion_handler;
 pub mod metrics;
 
-#[cfg(test)]
-pub(crate) mod tests;
-
 type CheckpointExecutionBuffer = FuturesOrdered<
     JoinHandle<(
         VerifiedCheckpoint,