Fixes #1410.

removes references to get_user_attributes, replaces calls to this plugin's own get_user_meta method with WP core's get_user_meta function
removes references to delete_user_attributes, replaces calls to this plugin's own delete_user_meta method with WP core's delete_user_meta function
removes references to update_user_attributes, replaces calls to this plugin's own update_user_meta method with WP core's update_user_meta function

Checklist

[ ] ~Project documentation has been updated to reflect the changes in this pull request, if applicable.~ not really applicable ❓
[ ] I have tested the changes in the local development environment (see contributing.md).
- I used this code while developing a custom connector, and did not observe any errors, but my connector didn't directly interface with user meta functions
[ ] I have added phpunit tests.
- ❓ I debated removing this function's get_user_meta, delete_user_meta, and update_user_meta methods, but decided to leave them in in case third-party code uses these functions. For that reason, I left these tests in places. Does that make sense?

Release Changelog

Fix: Removes *_user_attributes function calls, in response to WP VIP deprecating those functions. This plugin's *_user_meta functions are still available for third-party plugins to call.

[ ] 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.