Duplicate base checking code already covers check for opt base != XInterface

Change-Id: Icd47d87f93641a0b24a96a4be8d3d120f0405f2d

idlc/test/parser/interfaceinheritance.tests

@@ -269,3 +269,11 @@ interface Derived {
     interface Hidden;
     interface Base;
 };
+
+
+EXPECT FAILURE "interfaceinheritance.tests 30":
+interface Base {};
+interface Derived {
+    interface Base;
+    [optional] interface com::sun::star::uno::XInterface;
+};

unoidl/source/sourceprovider-parser.y

@@ -4113,7 +4113,6 @@ bool SourceProviderInterfaceTypeEntityPad::addDirectBase(
         addOptionalBaseMembers(
             location, yyscanner, data, base.name, base.entity);
     }
-    //TODO: check that opt base != XInterface
     (optional ? directOptionalBases : directMandatoryBases).push_back(base);
     return true;
 }