Flatten ReactCxxPlatform timer registry directory structure (#56536)

Summary:
Pull Request resolved: #56536

Move `PlatformTimerRegistryImpl.{h,cpp}` from `platform/cxx/react/runtime/` to the top level of `ReactCxxPlatform/react/runtime/`. The `platform/cxx/` nesting was an artifact of the multi-platform build setup that is no longer needed since `:timer_registry` is a standalone target.

- Remove dead `platform/cxx/react/runtime/*` glob patterns from `:runtime` BUCK target
- Remove `platform/cxx/` include directory from CMakeLists.txt
- Exported header path unchanged: `react/runtime/PlatformTimerRegistryImpl.h`

Changelog: [Internal]

Reviewed By: christophpurrer

Differential Revision: D101811797

Author: javache

packages/react-native/ReactCxxPlatform/react/runtime/CMakeLists.txt

@@ -8,13 +8,12 @@ set(CMAKE_VERBOSE_MAKEFILE on)
 
 include(${REACT_COMMON_DIR}/cmake-utils/react-native-flags.cmake)
 
-file(GLOB react_cxx_platform_react_runtime_SRC CONFIGURE_DEPENDS *.cpp platform/cxx/react/runtime/*.cpp)
+file(GLOB react_cxx_platform_react_runtime_SRC CONFIGURE_DEPENDS *.cpp)
 add_library(react_cxx_platform_react_runtime OBJECT ${react_cxx_platform_react_runtime_SRC})
 
 target_include_directories(react_cxx_platform_react_runtime
     PUBLIC
-      ${REACT_CXX_PLATFORM_DIR}
-      ${CMAKE_CURRENT_SOURCE_DIR}/platform/cxx/)
+      ${REACT_CXX_PLATFORM_DIR})
 
 target_link_libraries(react_cxx_platform_react_runtime
       bridgeless