This PR removes the feature flag for ECharts. ECharts feature needs to
enabled by default and doesn't need to be behind a feature flag anymore.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
- **New Features**
- Enhanced the Chart Widget by integrating Custom ECharts as a standard
option.
- **Bug Fixes**
- Addressed the Chart Widget selection issue by marking the deprecated
chart type accordingly.
- **Refactor**
- Streamlined Chart Widget properties and configuration for better
usability.
- Improved database query performance with new indexes.
- **Tests**
- Updated end-to-end tests to align with the Chart Widget changes.
- **Chores**
- Removed obsolete feature flags related to charting functionalities.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
