MSVC: Fix regression that links shared libraries with bogus -shared flag

Clear `CMAKE_SHARED_LIBRARY_CREATE_<LANG>_FLAGS` since no special
link flags are needed to create a DLL with MSVC toolchains.
Our shared library creation rules never referenced that placeholder,
so this variable was never used until commit 951e4d3f62 (Ninja,Makefile:
Move builtin linker flags to <LINK_FLAGS> placeholder, 2025-06-09,
v4.1.0-rc1~37^2~2) taught the generators to look it up directly.

This was not previously noticed because the `-shared` flag
is ignored unless the `-WX` flag enables an error on it.

Fixes: #27106

Modules/Platform/Windows-MSVC.cmake

@@ -393,6 +393,7 @@ macro(__windows_compiler_msvc lang)
     set(_CMAKE_VS_LINK_DLL "<CMAKE_COMMAND> -E vs_link_dll --msvc-ver=${MSVC_VERSION} --intdir=<OBJECT_DIR> --rc=<CMAKE_RC_COMPILER> --mt=<CMAKE_MT> --manifests <MANIFESTS> -- ")
     set(_CMAKE_VS_LINK_EXE "<CMAKE_COMMAND> -E vs_link_exe --msvc-ver=${MSVC_VERSION} --intdir=<OBJECT_DIR> --rc=<CMAKE_RC_COMPILER> --mt=<CMAKE_MT> --manifests <MANIFESTS> -- ")
   endif()
+  set(CMAKE_SHARED_LIBRARY_CREATE_${lang}_FLAGS "")
   if(CMAKE_SYSTEM_NAME STREQUAL "WindowsKernelModeDriver")
     set(_DLL_DRIVER "-driver")
   else()