RFR: 8260449: Remove stale declaration of SATBMarkQueue::apply_closure_and_empty()

[Roman Kennke]

JDK-8258742 removed apply_closure_and_empty(), but curiously JDK-8260263 reintroduced its declaration (but no definition).

Testing:
 - [x] build fastdebug/release on Linux/x86_64
 - [x] tier1

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Commit messages:
 - 8260449: Remove stale declaration of SATBMarkQueue::apply_closure_and_empty()

Changes: https://git.openjdk.java.net/jdk/pull/2242/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2242&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260449
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2242.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2242/head:pull/2242

PR: https://git.openjdk.java.net/jdk/pull/2242