Use Threadlocal.remove() instead of Threadlocal.set(null)

[Seungpang]

Hi Team,

This PR improves memory management by replacing instances of ThreadLocal.set(null) with ThreadLocal.remove() and removing the unnecessary ThreadLocal.set(null) in the constructor.

In the current implementation, ThreadLocal.set(null) is used to clear the ThreadLocal variable. This approach can leave the ThreadLocal variable in the thread's local variable map with a null value, potentially causing memory leaks. Using ThreadLocal.remove() instead properly removes the ThreadLocal variable from the thread's local variable map, preventing memory leaks.

Additionally, the constructor for JobFlowExecutor contains an unnecessary ThreadLocal.set(null), which is redundant because the ThreadLocal variable is already initialized to null by default.

[fmbenhassine]

LGTM. Rebased and merged as 3b9ceea. Thank you for your contribution!