Delete now-unused model classes Contact and Message

src/main/kotlin/org/briarproject/briar/desktop/paul/model/Contact.kt

-package org.briarproject.briar.desktop.paul.model
-
-data class Contact(
-    val name: String,
-    val online: Boolean,
-    val profile_pic: String,
-    val last_heard: String,
-    val privateMessages: List<Message>
-)

src/main/kotlin/org/briarproject/briar/desktop/paul/model/Message.kt

-package org.briarproject.briar.desktop.paul.model
-
-data class Message(
-    val from: String?,
-    val message: String,
-    val time: String,
-    val delivered: Boolean,
-    // val read: Boolean,
-)