Remove terminate_on_shutdown

This existed for the use of NPAPI, and the last caller passing `false` was removed in 2016 in http://crrev.com/385358.

The concept still exists for sanitizer builds, which is why
`terminate_child_on_shutdown_` was not removed.

Change-Id: Iebd980dba4e6ddbff26760d80644f3e2826a8207
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7004086
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Auto-Submit: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1524540}