Remove dead code for biblio for #1841 (#1875)

A community member reported the obsolete references to back-matter/resource/citation biblio element. The documentation that led to the bug report has also since been commented out, this commit will remove both commented out model and documentation.

src/metaschema/oscal_metadata_metaschema.xml

@@ -536,23 +536,7 @@
  <assembly ref="link" max-occurs="unbounded">
  <group-as name="links" in-json="ARRAY"/>
  </assembly>
-
-<!--
- <define-assembly name="biblio">
- <formal-name>Bibliographic Definition</formal-name>
- <description>A container for structured bibliographic information. The model of this information is undefined by OSCAL.</description>
- <model>
- <any/>
- </model>
- </define-assembly>
--->
  </model>
-<!--
- <remarks>
- <p>The <code>text</code> is used to define the endnote text, without any required bibliographic structure. If structured bibliographic data is needed, then the <code>biblio</code> can be used for this purpose.</p>
- <p>A <code>biblio</code> can be used to capture a structured bibliographical citation in an appropriate format.</p>
- </remarks>
--->
  </define-assembly>
  <define-assembly name="rlink" max-occurs="unbounded">
  <formal-name>Resource link</formal-name>