Revert "[heap] Promote young objects by default in MC"

This reverts commit 5e97378f.

Reason for revert: Caused multiple regressions.

Original change's description:
> [heap] Promote young objects by default in MC
> 
> Start experiment to promote all young live objects during mark-compact.
> 
> The last CL https://crrev.com/c/1879938 got reverted because of a flaky
> test, see v8:9192.
> 
> Change-Id: I16897f45fffeafbb7e70c21899976a4c026e69ba
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1903432
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#64832}

TBR=ulan@chromium.org,dinfuehr@chromium.org
Bug: chromium:1023308, chromium:1022708

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Ie551f0765fb54a36e52c20da8b026e2c0ebf0451
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1906385Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64882}

src/flags/flag-definitions.h

@@ -356,7 +356,7 @@ DEFINE_BOOL(assert_types, false,
 DEFINE_BOOL(allocation_site_pretenuring, true,
             "pretenure with allocation sites")
 DEFINE_BOOL(page_promotion, true, "promote pages based on utilization")
-DEFINE_BOOL(always_promote_young_mc, true,
+DEFINE_BOOL(always_promote_young_mc, false,
             "always promote young objects during mark-compact")
 DEFINE_INT(page_promotion_threshold, 70,
            "min percentage of live bytes on a page to enable fast evacuation")