Fix duplicated react/utils prefix in React-utils static header layout

The platformios subspec inherits header_dir "react/utils" from the parent
spec, and maps its headers relative to "platform/ios" — a directory whose
own layout already starts with react/utils. In a non-frameworks build both
layers are applied in sequence, so the platform headers land at
Pods/Headers/{Public,Private}/React-utils/react/utils/react/utils/*.h.

Because of that, #include <react/utils/LowPriorityExecutor.h> — reached from
the React/Utils.h umbrella, which several modules now include — cannot be
resolved and RNTester fails to build in the default static configuration:

  Pods/Headers/Public/React-utils/React/Utils.h:36:10: fatal error:
  'react/utils/LowPriorityExecutor.h' file not found

Clear the inherited header_dir on the subspec instead of moving the mapping
directory: the mapping already contributes the react/utils prefix, and
header_dir does not take part in the framework header copy path, so the
USE_FRAMEWORKS layout (React_utils.framework/Headers/react/utils/*.h) is
unchanged.

Author: zhongwuzw

packages/react-native/ReactCommon/react/utils/React-utils.podspec

@@ -58,6 +58,7 @@ Pod::Spec.new do |s|
 
   s.subspec "platformios" do |ss|
     ss.source_files        = podspec_sources("platform/ios/**/*.{m,mm,cpp,h}", "platform/ios/**/*.h")
+    ss.header_dir          = ""
     ss.header_mappings_dir = "platform/ios"
   end