xhtml_style: remove timestamp from ckeditor URLs

This timestamp break officejs offline capabilities.

This reaplies 40f6c8fe ([erp5_xhtml_style] ckeditor: drop hardcoded
timestamp parameter in URL, 2017-10-16) on the updated CKEditor

This was done with:

    find product/ERP5/bootstrap/erp5_xhtml_style/SkinTemplateItem/portal_skins/erp5_ckeditor/ckeditor/ -type f  | xargs sed -i 's/"M6K9"/""/g'

M6K9 was the timestamp for this build, that we can see at the top of
ckeditor.js