Revert "cppgc: young-gen: Enable cppgc_enable_young_generation by default"

This reverts commit 31009706.

Reason for revert: UBSan failure: https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux64%20UBSan/21069/overview

Original change's description:
> cppgc: young-gen: Enable cppgc_enable_young_generation by default
>
> Oilpan Young Generation is now controlled by the runtime flag
> --cppgc-young-generation.
>
> Bug: chromium:1029379
> Change-Id: I9ded9637f43a2f86993cff898cd7f272a051ae3c
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3616728
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Anton Bikineev <bikineev@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#80312}

Bug: chromium:1029379
Change-Id: I18ac696380df5f77d0978072b8e5af2f2e305994
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3620839
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#80318}

BUILD.gn

@@ -790,9 +790,6 @@ if (cppgc_enable_object_names) {
 }
 if (cppgc_enable_caged_heap) {
   enabled_external_cppgc_defines += [ "CPPGC_CAGED_HEAP" ]
-
-  # Always enable young generation compile time flag if caged heap is enabled.
-  cppgc_enable_young_generation = true
 }
 if (cppgc_enable_young_generation) {
   enabled_external_cppgc_defines += [ "CPPGC_YOUNG_GENERATION" ]