RFR: 8272343: Remove MetaspaceClosure::FLAG_MASK

[Ioi Lam]

Please review this simple fix. `MetaspaceClosure::FLAG_MASK` is no longer needed after https://bugs.openjdk.java.net/browse/JDK-8272107

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Commit messages:
 - 8272343: Remove MetaspaceClosure::FLAG_MASK

Changes: https://git.openjdk.java.net/jdk/pull/5245/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5245&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272343
  Stats: 21 lines in 3 files changed: 0 ins; 17 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5245.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5245/head:pull/5245

PR: https://git.openjdk.java.net/jdk/pull/5245