Clean-ups:
 * Remove the detaching of persistent handles from the LocalHeap if the
main thread will not get the handles from the background thread.
 * Remove unused isolate member.
 * Make members private/protected as needed.

Bug: v8:7790
Change-Id: I23bf4a41124bd04d4a848edfa1ef8f9e8e77182c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2463234Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70442}