Summary:
Use transpiled `react-native-codegen` scripts. By avoiding use of flow-node, we no longer need to upgrade flow-remove-types in order to run the native modules codegen script.
The interface for the script remains the same:
```
./scripts/generate-native-modules-specs.sh [optionalOutputDir]
```
Changelog: [Internal]
Reviewed By: TheSavior
Differential Revision: D21629705
fbshipit-source-id: 7714a67e36c8151a3b0b49285b6a6c93a525d7bc
Héctor Ramos