CMake: Remove duplicate platform info includes

It looks like something was merged in a funny way resulting in the
platform info module being included twice in the top-level CMakeLists
files. Removing one of them.

Author: etcwilde

CMakeLists.txt

@@ -24,8 +24,6 @@ if(NOT SWIFT_SYSTEM_NAME)
   endif()
 endif()
 
-include(PlatformInfo)
-
 set(CMAKE_Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift)
 set(CMAKE_Swift_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY MultiThreadedDLL)