[FLINK-6597][streaming] Cleanup WindowOperatorTest useless imports

[sunjincheng121]

Don't miss doing any good thing no matter how insignificant it looks.

• General

  • The pull request references the related JIRA issue ("[FLINK-6597][streaming] Cleanup WindowOperatorTest useless imports")
  • The pull request addresses only one issue
  • Each commit in the PR has a meaningful commit message (including the JIRA id)

• Documentation

  • Documentation has been added for new functionality
  • Old documentation affected by the pull request has been updated
  • JavaDoc for public methods has been added

• Tests & Build

  • Functionality added by the pull request is covered by tests
  • mvn clean verify has been executed successfully locally or a Travis build has passed

[fhueske]

+1 to merge

[greghogan]

@sunjincheng121 thanks for this PR. @fhueske what do you think of enabling checkstyle's includeTestSourceDirectory?

[greghogan]

Perhaps we should enable checkstyle on test sources when upgrading to the strict checkstyle.

[fhueske]

@greghogan that would make sense to me. Not sure why we excluded the tests in the first place.