[v5] Remove recursive header search from ios podspec

I believe following commit introduces an issue fro cocoapods users (of whoom there will be many now that react-native uses cocoapods) b85153d#diff-1b2b06e7c58d8929c91f917ce51a7454R28

The gist of it, that it introduces back this issue firebase/firebase-ios-sdk#1916
It is due to some header include in gRPC-C++ library that recursive header search will try to include, but won't find, hence causing build error.

cc @MoOx as he authored original PR, I am not sure if removing this will cause any issues, so wanted to discuss it with you.

Author: iljadaderko

ios/RNFirebase.podspec

@@ -23,8 +23,4 @@ Pod::Spec.new do |s|
   end
   # allow this package to be used with use_frameworks!
   s.static_framework = true
-  # fix recursive header flag being skipped by cocoapods when using this as a framework
-  s.xcconfig = {
-    'HEADER_SEARCH_PATHS' => '${PODS_ROOT}/Headers/Public/**'
-  }
 end