build: remove unnecessary CXX_STANDARD for Windows

The entire project is now at C++14.  Windows had to adopt C++14 earlier
to support processing the Windows SDK headers.  This is no longer
necessary, adopt the global settings instead.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -517,7 +517,6 @@ function(add_swift_host_library name)
     endif()
 
     set_target_properties(${name} PROPERTIES
-      CXX_STANDARD 14
       NO_SONAME YES)
   endif()