This PR removes/simplifies some APIs that were leftover after moving the
agent settings from the agent panel into the agent settings UI.
Behavior/UI should be identical.

- Removed `ConfigurationViewTargetAgent` since only a single variant was
used
- Removed `configuration_view` and `configuration_view_v2` and replaced
it with `settings_view`
- Removed all the custom configuration views that were replaced by the
API key view abstraction
- Removed `intitial_title` and `initial_description` and moved it to
`ProviderSettingsView`

Release Notes:

- N/A