After flags are frozen, this will not work any more. It's also not
required, as flags cannot be accessed after teardown anyway.
This CL changes that to only release the memory of dynamically allocated
string flags, which is something we still need to do after
write-protecting the flags anyway.

R=tebbi@chromium.org

Bug: v8:12887
Change-Id: Iff0e3845cbd91fb59878b2ed36a44d6df00572f4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695379Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81030}