Clean up `Indicator` (#11275)

This PR cleans up the `Indicator` component:

- Renamed `IndicatorStyle` to `IndicatorKind` and made it private.
- Fixed `Indicator::bar()` to construct an indicator using the right
`IndicatorKind`.
- Removed the `IndicatorIcon`, since we didn't actually end up using it.

Release Notes:

- N/A