[docs] update interfaces documentation (#741)

We currently support interfaces implementing other interfaces so removing old docs.

Author: dariuszkuc

docs/writing-schemas/interfaces.md

@@ -90,42 +90,3 @@ sealed class Pet(val name: String) {
     class Cat(name: String, val livesRemaining: Int) : Pet(name)
 }
 ```
-
-### Known Issues
-> We currently do not support multiple levels of interfaces in a schema. We are waiting until graphql-java supports the newly added feature to the GraphQL spec. [See 589](https://github.com/ExpediaGroup/graphql-kotlin/issues/589). If you do have multiple interfaces you will have to either combine them into a single interface or ignore all the parent interfaces by marking them private or using `@GraphQLIgnore`.
-
-#### Invalid Schema
-```kotlin
-interface FirstLevel {
-    val id: String
-}
-
-interface SecondLevel : FirstLevel {
-    val name: String
-}
-
-class MyClass(override val id: String, override val name: String) : SecondLevel
-```
-
-#### Valid Schema
-```kotlin
-@GraphQLIgnore
-interface FirstLevel {
-    val id: String
-}
-
-interface SecondLevel : FirstLevel {
-    val name: String
-}
-
-class MyClass(override val id: String, override val name: String) : SecondLevel
-```
-OR
-```kotlin
-interface FirstLevel {
-     val id: String
-     val name: String
-}
-
-class MyClass(override val id: String, override val name: String) : FirstLevel
-```