Fix "size reset" problem

The issue was :
When an image has a defined size, and when you clear the height and width fields, the removal of height and width tags is not remembered (because tags like "_fck_mw_width" are not removed).

(This issue was distinct and don't interfere with https://github.com/Mediawiki-wysiwyg/WYSIWYG-CKeditor/pull/17/files )