Removed some incomplete code for the Windows platform.

Please, refer to the comment below.
ros2/rclcpp#2205 (comment)

Signed-off-by: Shoji Morita <s-morita@esol.co.jp>

Author: smorita-esol

CMakeLists.txt

@@ -35,9 +35,6 @@ target_include_directories(${PROJECT_NAME} PUBLIC
 if(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
   target_sources(${PROJECT_NAME} PRIVATE
     src/threads/posix_thread.cpp)
-elseif(WIN32)
-  target_sources(${PROJECT_NAME} PRIVATE
-    src/threads/windows_thread.cpp)
 endif()
 if(WIN32)
   target_compile_definitions(${PROJECT_NAME}

include/rcpputils/threads.hpp

@@ -17,8 +17,6 @@
 
 #if defined(__linux__)
 #include "rcpputils/threads/posix/thread.hpp"
-#elif defined(_WIN32)
-#include "rcpputils/threads/windows/thread.hpp"
 #else
 #include "rcpputils/threads/std/thread.hpp"
 #endif