CMakeDetermineCompilerId: Fix VS Itanium platform name (#15889)

VS expects the platform to be just `Itanium`, so drop the incorrect
special case mapping it to `ia64`.  This platform name has been wrong
since the logic was added by commit v2.8.10~148^2~8 (VS: Detect the
compiler id and tool location, 2012-08-16).

Modules/CMakeDetermineCompilerId.cmake

@@ -182,9 +182,6 @@ Id flags: ${testflags}
       set(v 6)
       set(ext dsp)
     endif()
-    if("${id_platform}" STREQUAL "Itanium")
-      set(id_platform ia64)
-    endif()
     if(CMAKE_VS_PLATFORM_TOOLSET)
       if(CMAKE_VS_PLATFORM_NAME STREQUAL "Tegra-Android")
         set(id_toolset "<NdkToolchainVersion>${CMAKE_VS_PLATFORM_TOOLSET}</NdkToolchainVersion>")