Create From Usage: Generate Unit-typed declaration if result is unused

#KT-5903 Fixed

idea/testData/quickfix/createFromUsage/createFunction/binaryOperations/afterPlusAssignOnUserType.kt

@@ -1,7 +1,7 @@
 // "Create function 'plusAssign' from usage" "true"
 
 class A<T>(val n: T) {
-    fun plusAssign(arg: T): Unit {
+    fun plusAssign(arg: T) {
         throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
     }
 }