Revert "GH-126491: GC: Mark objects reachable from roots before doing cycle collection (GH-126502)"

[hugovk]

This reverts commit b0fcc2c.

• Issue: Mark all objects reachable from roots as live before doing main cyclic GC pass #126491

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @hugovk for commit 2ac5c48 🤖

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[hugovk]

Merged because it's 3.14.0a2 release day and there's two failing tier-1 buildbots, two failing tier-2, and six failing tier-3. Tier-1 need fixing/reverting immediately and tier-2 within 24 hours. The first failing tier-1 was 14 hours ago and tier-2 was 18 hours ago.

• https://buildbot.python.org/#/release_status
• https://peps.python.org/pep-0011/