Remove references to deprecated filter.

[ethanclevenger91]

Fixes #1343 .

Describe your approach and how it fixes the issue.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• 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

• This pull request is to the master branch.
• 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.

[ethanclevenger91]

I honestly don't exactly understand what the purpose of this array of class-specific constants is. They seem to be re-implementations of available PHP core constants.

I updated references to FILTER_SANITIZE_STRING to use FILTER_SANITIZE_SPECIAL_CHARS, which is keyed to htmlspecialchars, which is the documented alternative