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

This reverts commit e2376aed.

Reason for revert: Causes failures on multiple bots of regress-678917
https://ci.chromium.org/p/v8/builders/ci/V8%20Linux64%20-%20cfi/22459

Original change's description:
> [heap] Promote young objects by default in MC
> 
> Start experiment with promoting all young live objects during
> mark-compact. This CL sets always_promote_young_mc to true by default.
> 
> Change-Id: I306527ad2a361fc3e0edcf684c5ccec2b1d6e491
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1879938
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#64630}

TBR=ulan@chromium.org,dinfuehr@chromium.org

Change-Id: I439dfee84a6795caa47f4c9a869ccef44a46a73f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1891512Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64664}

src/flags/flag-definitions.h

@@ -355,7 +355,7 @@ 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")