Refactor extension point schema

Issue-Ref: 4030
Issue-Url: https://af3-developer.fortiss.org/issues/4030

Signed-off-by: Tiziano Munaro <munaro@fortiss.org>

org.fortiss.af3.exploration.ui/schema/externalDSEExtensionProvider.exsd

@@ -49,15 +49,10 @@
 
    <element name="externalDSEExtensionProvider">
       <complexType>
-         <sequence>
-            <element ref="externalDSEExtensionProvider" minOccurs="1" maxOccurs="unbounded"/>
-         </sequence>
          <attribute name="externalDSEExtensionProvider" type="string" use="required">
             <annotation>
                <documentation>
                   
-                  
-               
                </documentation>
                <appInfo>
                   <meta.attribute kind="java" basedOn=":org.fortiss.af3.exploration.ui.perspective.extension.IExternalDSEExtensionProvider"/>