CiviEvent Dashboard and Manage Events: clean up disused url template vars

[agh1]

Overview

This just cleans up the variables that store the URLs for event feeds. These were generated in the template previously, but as of #17282 they're generated in the page class and stored differently. I simply forgot to remove the old way when I made the earlier change.

Before

Four Smarty variables are set but never used on the CiviEvent Dashboard and Manage Events.

After

They aren't set.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).