move clang plugin that's no longer needed

But still keep it in case it'd be needed again, or simply
as a starting reference.

Change-Id: If519e1320e1bd6dce7746b57172324b33504051e

compilerplugins/clang/store/README

+This plugin actions are not used. They are still kept in case they would be useful again
+(they can be activated again by simply moving them back in the clang/ source directory)
+or simply as a reference when writing new plugins.