Fix an apparent crediting error

It seems like G. Luff got put here as well as on the hyper-schema
spec when the project restarted, which (based on past credits and
the known authorship of the first post-restart draft) was an
error.

He is still listed in the acknowledgements with other past
major contributors to the overall project.

Author: handrews

jsonschema-validation.xml

@@ -54,17 +54,6 @@
             </address>
         </author>
 
-        <author fullname="Geraint Luff" initials="G" surname="Luff">
-            <address>
-                <postal>
-                    <street></street>
-                    <city>Cambridge</city>
-                    <country>UK</country>
-                </postal>
-                <email>[email protected]</email>
-            </address>
-        </author>
-
         <date year="2019"/>
         <workgroup>Internet Engineering Task Force</workgroup>
         <keyword>JSON</keyword>