RFR: 8367507: Parallel: Remove PSPromotionManager::drain_stacks_depth

[Albert Mingkun Yang]

Trivial renaming a method so that we can remove one of two methods with identical implementations, `drain_stacks` and `drain_stacks_depth`.

Test: tier1

-------------

Commit messages:
 - pgc-stack-merge

Changes: https://git.openjdk.org/jdk/pull/27252/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27252&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367507
  Stats: 8 lines in 3 files changed: 0 ins; 4 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/27252.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27252/head:pull/27252

PR: https://git.openjdk.org/jdk/pull/27252