Remove nullness suppression in TimestampCombiner

[nitinware]

Progress on #20507 (sub-task of umbrella #20497).

The class-level @SuppressWarnings({"nullness", "keyfor"}) on
TimestampCombiner has no matching Checker Framework diagnostic:
the enum has no fields, and Iterables.get(timestamps, 0) in
END_OF_WINDOW.combine is preceded by checkArgument(size > 0).
Compiling sdks:java:core with -PenableCheckerFramework is clean
after removing the annotation, so no other changes are needed.

Verified locally against sdks:java:core:

• spotlessJavaCheck, checkstyleMain
• compileJava -PenableCheckerFramework (clean rebuild)
• WindowingStrategyTranslationTest, WindowingTest

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[github-actions]

Assigning reviewers:

R: @chamikaramj for label java.

Note: If you would like to opt out of this review, comment assign to next reviewer.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).

[nitinware]

@kennknowles thanks for the review and merge! Appreciate it.