iOS: Generate FBReactNativeSpec ObjC++ files at build time

Summary:
Removes the generated FBReactNativeSpec files from source control.

## Generating FBReactNativeSpec files

The files will be generated automatically by CocoaPods when the `RNTesterPods` Xcode workspace is generated:

```
cd packages/rn-tester
pod install
```

The spec files can be re-generated by invoking the script directly:
```
./scripts/generate-native-modules-specs.sh
```

Changelog: [Internal]

Reviewed By: RSNara

Differential Revision: D24261167

fbshipit-source-id: acb7ac856e5e519932e4f587f79e24f49cd84a91

Author: hramos

.gitignore

@@ -100,6 +100,7 @@ package-lock.json
 !/packages/rn-tester/Pods/__offline_mirrors__
 
 # react-native-codegen
+/Libraries/FBReactNativeSpec/FBReactNativeSpec
 /packages/react-native-codegen/lib
 /ReactCommon/fabric/components/rncore/
 /schema-native-modules.json