RFR: 8253220: Epsilon: clean up unused code/declarations

[Aleksey Shipilev]

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

Testing: Linux x86_64 fastdebug gc/epsilon

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Commit messages:
 - 8253220: Epsilon: clean up unused code/declarations

Changes: https://git.openjdk.java.net/jdk/pull/199/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=199&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253220
  Stats: 2 lines in 2 files changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/199.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/199/head:pull/199

PR: https://git.openjdk.java.net/jdk/pull/199