Remove unneeded date comment in gradle-wrapper.properties

[asashour]

As the comment does not add any value.

Fixes #103616

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[blasten]

Thanks! there are a few more entries https://github.com/flutter/flutter/search?q=CEST+2018

[asashour]

Thanks, done.

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• Please get at least one approved review if you are already a member or two member reviews if you are not a member before re-applying this label. Reviewers: If you left a comment approving, please use the "approve" review action instead.

[christopherfujino]

CI is failing on your PR because there were problems upstream on master at the time you branched. If you rebase upstream, CI will re-run and checks should pass. Sorry about the trouble and thanks for contributing!