Codegen: Remove generate-specs.sh

Summary:
First part of the codegen script cleanup effort. Everything that was done in generate-specs.sh is now part of the CocoaPods recipe (e.g. codegen method in `react_native_pods.rb`).

Now that `generate-specs.sh` has been removed, the codegen may still be invoked manually for test purposes like so:

```
cd react-native

# Generate Schema - do this whenever your JS specs change
node packages/react-native-codegen/lib/cli/combine/combine-js-to-schema-cli.js <output_file_schema_json> <javascript_sources_dir>

# Generate native interfaces for iOS (use schema.json generated by previous step)
node scripts/generate-specs-cli.js ios <output_file_schema_json> <output_dir> <library_name>
```

Changelog: [Internal]

Reviewed By: fkgozali

Differential Revision: D30648067

fbshipit-source-id: 29688e0aac5496886657db82becb05bc8da076c9