tags: remove normative text not found in the rest of the document

The rule of First Come First Served involves that the name can't be found twice
and (hopefully) the same thing can't be defined in 2 different ways.

Author: robUx4

cellar-tags/tags_iana.md

@@ -11,10 +11,8 @@ a Change Controller, and
 an optional Reference to a document describing the Element ID.
 
 The Name corresponds to the value stored in the `TagName` element.
-A Tag Name **MUST** only be found once in the IANA registry.
-Two Tag Names **MUST NOT** have the same semantic meaning.
 The Name is written in all latin capital letters, numbers and the underscore character '_'
-as defined in (#tag-formatting). The Name **MUST NOT** start with the underscore character '_'.
+as defined in (#tag-formatting). The Name must not start with the underscore character '_'.
 
 The Type corresponds to which element will be stored the tag value.
 There can be 3 values for the Type: