Remove isHermesV1 from Babel preset (#56754)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56754

Hermes V1 (Static Hermes) is now always enabled. Remove the isHermesV1 variable and inline its effects:
- enableRegenerator is always based on dev mode (was isHermesV1 && dev)
- preserveClasses is always true (was isHermesV1)

Also update the comments to drop the 'V1' framing.

## Changelog:
[Internal] -

Reviewed By: cortinico

Differential Revision: D104649639

fbshipit-source-id: 70d4cbc0ad365df551b744e3cc16e880ce41c90c