Revert "Merge pull request #2008 from dlech/CustomAttributeData"

This reverts commit 2775839f66faf67803efd96af7aa075888a1e09b, reversing
changes made to 9cf3500204c70d0cf69584a2c47c2c1591f198b0.

mcs/class/corlib/System.Reflection/CustomAttributeData.cs

@@ -131,7 +131,7 @@ namespace System.Reflection {
 			return MonoCustomAttrs.GetCustomAttributesData (target);
 		}
 
-		public virtual Type AttributeType {
+		public Type AttributeType {
 			get { return ctorInfo.DeclaringType; }
 		}