Defragment Codegen in OSS between Old and New Architecture (#41500)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41500

Right now, the old architecture uses Codegen in a slightly different way w.r.t. the New Architecture.
In the Old Architecture, codegen is used to generate some basic TM and components that are part of Core.

Both architectures use the same scripts that actually generates the code, but they are invoked differently.

This is causing some maintenance costs that we would like to reduce.

## Changelog:
[Internal] - Defragment how Codegen is run between old and new architecture

Reviewed By: dmytrorykun

Differential Revision: D51349874

fbshipit-source-id: 188d3ed436a30a77bd42a26306d4a08666d3a00b

packages/rn-tester/NativeModuleExample/ScreenshotManager.podspec

@@ -23,13 +23,4 @@ Pod::Spec.new do |s|
   s.requires_arc    = true
 
   install_modules_dependencies(s)
-
-  # s.dependency "..."
-
-  # Enable codegen for this library
-  use_react_native_codegen!(s, {
-    :react_native_path => "../../react-native",
-    :js_srcs_dir => "./",
-    :library_type => "modules",
-  })
 end