RFR: 8280823: Remove NULL check in DumpTimeClassInfo::is_excluded

[Ioi Lam]

Please review this trivial change. The NULL check is not needed anymore because the ``DumpTimeClassInfo::_klass` field is never set to NULL.

Passed Oracle IT tiers 1~4

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Commit messages:
 - 8280823: Remove NULL check in DumpTimeClassInfo::is_excluded

Changes: https://git.openjdk.java.net/jdk/pull/7249/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7249&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280823
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7249.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7249/head:pull/7249

PR: https://git.openjdk.java.net/jdk/pull/7249