ExternalData: Remove unused private interface

The 'store' action was part of an older design and is no longer needed.
Remove it.

Modules/ExternalData.cmake

@@ -690,16 +690,6 @@ elseif("${ExternalData_ACTION}" STREQUAL "local")
     endif()
   endforeach()
   _ExternalData_link_or_copy("${name}" "${file}")
-elseif("${ExternalData_ACTION}" STREQUAL "store")
-  foreach(v dir file)
-    if(NOT DEFINED "${v}")
-      message(FATAL_ERROR "No \"-D${v}=\" value provided!")
-    endif()
-  endforeach()
-  if(NOT DEFINED algo)
-    set(algo "MD5")
-  endif()
-  _ExternalData_compute_hash(hash "${algo}" "${file}")
 else()
   message(FATAL_ERROR "Unknown ExternalData_ACTION=[${ExternalData_ACTION}]")
 endif()