RFR: 8281585: Remove unused imports under test/lib and jtreg/gc

[Leo Korinth]

Remove unused imports under test/lib and jtreg/gc. They create lots of warnings if editing using an IDE. Tests in hotspot_gc passed.

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Commit messages:
 - 8281585: Remove unused imports under test/lib and jtreg/gc

Changes: https://git.openjdk.java.net/jdk/pull/7426/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7426&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281585
  Stats: 92 lines in 60 files changed: 0 ins; 92 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7426.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7426/head:pull/7426

PR: https://git.openjdk.java.net/jdk/pull/7426