Remove 's.static_framework = true' requirement for podspec (#25816)

Summary:
As part of the fix for https://github.com/facebook/react-native/issues/25349 I added `s.static_framework = true` to each podspec in repo (see https://github.com/facebook/react-native/pull/25619#discussion_r306993309 for more context).

This was required to ensure the existing conditional compilation with `#if RCT_DEV` and `__has_include` still worked correctly when `use_frameworks!` is enabled.

However, fkgozali pointed out that it would be ideal if we didn't have this requirement as it could make life difficult for third-party libraries.

This removes the requirement by moving `React-DevSupport.podspec` and `React-RCTWebSocket.podspec` into `React-Core.podspec` as subspecs. This means the symbols are present when `React-Core.podspec` is built dynamically so `s.static_framework = true` isn't required.

This means that any `Podfile` that refers to `React-DevSupport` or `React-RCTWebSocket` will need to be updated to avoid errors.

## Changelog

I don't think this needs a changelog entry since its just a refinement of https://github.com/facebook/react-native/pull/25619.
Pull Request resolved: https://github.com/facebook/react-native/pull/25816

Test Plan:
Check `RNTesterPods` still works both with and without `use_frameworks!`:

1. Go to the `RNTester` directory and run `pod install`.
2. Run the tests in `RNTesterPods.xcworkspace` to see that everything still works fine.
3. Uncomment the `use_frameworks!` line at the top of `RNTester/Podfile` and run `pod install` again.
4. Run the tests again and see that it still works with frameworks enabled.

Reviewed By: hramos

Differential Revision: D16495030

Pulled By: fkgozali

fbshipit-source-id: 2708ac9fd20cd04cb0aea61b2e8ab0d931dfb6d5

Libraries/PushNotificationIOS/React-RCTPushNotification.podspec

@@ -30,7 +30,6 @@ Pod::Spec.new do |s|
   s.source_files           = "*.{m}"
   s.preserve_paths         = "package.json", "LICENSE", "LICENSE-docs"
   s.header_dir             = "RCTPushNotification"
-  s.static_framework       = true
 
   s.dependency "React-Core/RCTPushNotificationHeaders", version
 end