heap: Remove redundant GCInfo::non_trivial_finalizer

Encode non-triviality in GCInfo::finalize being null.

Change-Id: I15fdab5a3561a447d87be7cadb50f15cd5f977ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1593308
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@google.com>
Cr-Commit-Position: refs/heads/master@{#656691}