RFR: 8261152: Refine the compiler/vectorapi/VectorRebracket128Test.java test

[王超]

Refine test VectorRebracket128Test.java as discussed here https://github.com/openjdk/jdk16/pull/139#discussion_r567796847

1, Explicit trigger gc in the test
2, Remove redundant imports
3, Remove weird java options

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Commit messages:
 - 8261152: Refine the compiler/vectorapi/VectorRebracket128Test.java test

Changes: https://git.openjdk.java.net/jdk/pull/2422/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2422&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261152
  Stats: 26 lines in 1 file changed: 17 ins; 7 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2422.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2422/head:pull/2422

PR: https://git.openjdk.java.net/jdk/pull/2422