Cleanup React-Core

Summary:
React-Core does not depends on any bit of ReactCommon, React-RCTFabric or React-NativeModuleApple, so I'm cleaning that up.

## Context
Last week I helped macOS to work with static framework.
When multiple platforms are specified, frameworks are build in two variants, the iOS and macOS one.

This break all the HEADER_SEARCH_PATHS as now we have to properly specify the base folder from which the search path is generated.
See also [this PR](microsoft#1967) where I manually make MacOS work with `use_framewroks!`

## Changelog:
[Internal] - Add helper function to create header_search_path

Differential Revision: D51030115

Author: Riccardo Cipolleschi

packages/react-native/React-Core.podspec

@@ -22,7 +22,6 @@ socket_rocket_version = '0.7.0'
 boost_compiler_flags = '-Wno-documentation'
 
 use_hermes = ENV['USE_HERMES'] == nil || ENV['USE_HERMES'] == '1'
-use_frameworks = ENV['USE_FRAMEWORKS'] != nil
 
 header_subspecs = {
   'CoreModulesHeaders'          => 'React/CoreModules/**/*.h',
@@ -40,8 +39,6 @@ header_subspecs = {
 
 frameworks_search_paths = []
 frameworks_search_paths << "\"$(PODS_CONFIGURATION_BUILD_DIR)/React-hermes\"" if use_hermes
-frameworks_search_paths << "\"${PODS_CONFIGURATION_BUILD_DIR}/ReactCommon\"" if use_frameworks
-frameworks_search_paths << "\"$(PODS_CONFIGURATION_BUILD_DIR)/React-RCTFabric\"" if use_frameworks
 
 header_search_paths = [
   "$(PODS_TARGET_SRCROOT)/ReactCommon",
@@ -51,15 +48,10 @@ header_search_paths = [
   "$(PODS_ROOT)/RCT-Folly",
   "${PODS_ROOT}/Headers/Public/FlipperKit",
   "$(PODS_ROOT)/Headers/Public/ReactCommon",
-  "$(PODS_ROOT)/Headers/Public/React-RCTFabric"
 ].concat(use_hermes ? [
   "$(PODS_ROOT)/Headers/Public/React-hermes",
   "$(PODS_ROOT)/Headers/Public/hermes-engine"
-] : []).concat(use_frameworks ? [
-  "$(PODS_CONFIGURATION_BUILD_DIR)/ReactCommon/ReactCommon.framework/Headers",
-  "$(PODS_CONFIGURATION_BUILD_DIR)/ReactCommon/ReactCommon.framework/Headers/react/nativemodule/core",
-  "$(PODS_CONFIGURATION_BUILD_DIR)/React-NativeModulesApple/React_NativeModulesApple.framework/Headers"
-] : []).map{|p| "\"#{p}\""}.join(" ")
+] : [])
 
 Pod::Spec.new do |s|
   s.name                   = "React-Core"