RFR: 8336661: Parallel: Remove stacks_empty assert in PSScavenge::invoke

[Albert Mingkun Yang]

Trivial removing redundant asserts.

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Commit messages:
 - pgc-remove-assert

Changes: https://git.openjdk.org/jdk/pull/20218/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20218&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336661
  Stats: 7 lines in 1 file changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20218.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20218/head:pull/20218

PR: https://git.openjdk.org/jdk/pull/20218