RFR: JDK-8277396: [TESTBUG] In DefaultButtonModelCrashTest.java, frame is accessed from main thread

[Alexey Ivanov]

It's a little cleanup: make sure `frame` is accessed from EDT only, remove unused variables and imports.

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Commit messages:
 - JDK-8277396: [TESTBUG] In DefaultButtonModelCrashTest.java, frame is accessed from main thread

Changes: https://git.openjdk.java.net/jdk/pull/6455/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6455&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277396
  Stats: 13 lines in 1 file changed: 4 ins; 5 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6455.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6455/head:pull/6455

PR: https://git.openjdk.java.net/jdk/pull/6455