ENH: Remove unused expatDllConfig.h references and associated file

This file, and the contents of this file (the definition of ITK_EXPAT_STATIC) are not
used anywhere in ITK.

Author: hjmjohnson

Modules/ThirdParty/Expat/src/expat/CMakeLists.txt

@@ -340,9 +340,6 @@ if (ITK_USE_UPSTREAM_CODE) # ITK UNUSED UPSTREAM CODE
 else () # ITK CHANGE START
   configure_file(expat_config.h.cmake "${ITK3P_EXPAT_BINARY_DIR}/expat_config.h")
 
-  configure_file(${ITK3P_EXPAT_SOURCE_DIR}/expatDllConfig.h.in
-      ${ITK3P_EXPAT_BINARY_DIR}/expatDllConfig.h)
-
   set(MANGLE_PREFIX itk_expat)
   configure_file(itk_expat_mangle.h.in
       "${ITK3P_EXPAT_BINARY_DIR}/itk_expat_mangle.h"