Remove references to deprecated filter.

[ethanclevenger91]

Fixes #1343 .

Describe your approach and how it fixes the issue.

Checklist

• [X] Project documentation has been updated to reflect the changes in this pull request, if applicable.
• [X] I have tested the changes in the local development environment (see contributing.md).
• [ ] I have added phpunit tests.

Release Changelog

• Change: Remove deprecated filter constant.

Release Checklist

• [X] This pull request is to the master branch.
• [X] Release version follows semantic versioning. Does it include breaking changes?
• [ ] Update changelog in readme.txt.
• [ ] Bump version in stream.php.
• [ ] Bump Stable tag in readme.txt.
• [ ] Bump version in classes/class-plugin.php.
• [ ] Draft a release on GitHub.

Change [ ] to [x] to mark the items as done.