[Storybook] Remove the se23 and shadow bevel toggle

[sophschneider]

Also clean up polaris-react/playground/DetailsPage.scss

### Tasks
- [x] Remove se23 toggle and any feature flag logic from `polaris-react/.storybook`
- [x] Remove se23 shadow bevel opt out toggle and any feature flag logic from `polaris-react/.storybook`
- [x] Remove any instances of `STORYBOOK_SE23='on'`
- [x] Remove feature flag story

[laurkim]

Partially addressed by 10242.
Remaining storybook clean up can happen with AppProvider consolidation.