Clean up unused waffle flags

[mrchrisadams]

In order to reduce some unecessary work when spinning up gitpod in our local environments we should clean up some old flags.

Specifically, we think we no longer need flags for:

1. the ProviderPortalHomeView - provider_request
2. directory listing - directory_listing
3. Hiding the old admin - hide_old_admin

[hanopcan]

https://trello.com/c/HeJ8r1uI/372-clean-up-unused-waffle-flags