RFR: 8297435: Remove unused CompactibleSpaceClosure

[Albert Mingkun Yang]

Trivial removing of dead code.

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

Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/11290/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11290&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297435
  Stats: 7 lines in 1 file changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11290.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11290/head:pull/11290

PR: https://git.openjdk.org/jdk/pull/11290