Removes APIs:
- MarkIndependent
- IsIndependent
- MarkActive
- RegisterExternalReference

All weak persistent handles are now treated as independent. Users of
traced handles should already use v8::EmbedderHeapTracer.

Bug: chromium:923361
Change-Id: Ic90a647fe2ce9db92197ad6560e4907290805592
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1578459
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60953}