chore(android): remove unused returnType field

core/tauri/mobile/android/src/main/java/app/tauri/annotation/PluginMethod.kt

@@ -5,4 +5,4 @@
 package app.tauri.annotation
 
 @Retention(AnnotationRetention.RUNTIME)
-annotation class PluginMethod(val returnType: String = "promise") { }
+annotation class PluginMethod

core/tauri/mobile/android/src/main/java/app/tauri/plugin/PluginMethodData.kt

@@ -13,7 +13,4 @@ class PluginMethodData(
 
   // The name of the method
   val name: String = method.name
-
-  // The return type of the method (see PluginMethod for constants)
-  val returnType: String = methodDecorator.returnType
 }