chore: removed obsolete feature flags

[AhtishamShahid]

Description

Removed following flags for notificiations:

notifications.show_notifications_tray notifications.enable_notifications_filters notifications.enable_coursewide_notifications notifications.enable_ora_staff_notifications discussions.enable_reported_content_notifications

Ticket

https://2u-internal.atlassian.net/browse/INF-1419

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.