fix: Remove frontend directory from prepare frontend output list (#17405)

Author: mshabarov

flow-plugins/flow-gradle-plugin/src/main/kotlin/com/vaadin/gradle/PrepareFrontendOutputProperties.kt

@@ -74,11 +74,6 @@ public class PrepareFrontendOutputProperties public constructor(project: Project
         return File(project.projectDir, "types.d.ts")
     }
 
-    @OutputDirectory
-    public fun getFrontendDirectory(): File {
-        return extension.frontendDirectory
-    }
-
     @OutputDirectory
     public fun getGeneratedTsFolder(): File {
         return extension.generatedTsFolder