RFR: 8253219: Epsilon: clean up unnecessary includes

[Aleksey Shipilev]

Static analysis shows a few #include statements in gc/epsilon that are not needed.

Testing: Linux x86_64 {release, fastdebug, slowdebug} x {+PCH, -PCH}

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Commit messages:
 - 8253219: Epsilon: clean up unnecessary includes

Changes: https://git.openjdk.java.net/jdk/pull/198/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=198&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253219
  Stats: 3 lines in 2 files changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/198.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/198/head:pull/198

PR: https://git.openjdk.java.net/jdk/pull/198