Fix LinkageType

src/de/steamwar/linkage/LinkageType.java

@@ -30,7 +30,7 @@ import java.util.Set;
 public interface LinkageType {
 
     Context context = LinkageProcessor.getContext();
-    default Optional<String> getPluginMain() {
+    default String getPluginMain() {
         return LinkageProcessor.getPluginMain();
     }