Mantis#1682. Thank you kindly, Sempuki for a patch that:

Move control of Mono.Addins from source attributes to external XML files.
This removes a lot of coupling of the source with Mono.Addins

OpenSim/Region/Application/IApplicationPlugin.cs

@@ -25,14 +25,10 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-using Mono.Addins;
 using OpenSim.Framework;
 
-[assembly : AddinRoot("OpenSim", "0.5")]
-
 namespace OpenSim
 {
-    [TypeExtensionPoint("/OpenSim/Startup")]
     public interface IApplicationPlugin : IPlugin
     {
         void Initialise(OpenSimBase openSim);