Remove Serializable interface from Source

No longer needed since we don't serialize it for the migration controller bundle.

(cherry picked from commit 7dd9a0211b)

app/src/main/java/eu/kanade/tachiyomi/source/Source.kt

@@ -5,7 +5,6 @@ import eu.kanade.tachiyomi.extension.ExtensionManager
 import eu.kanade.tachiyomi.source.model.Page
 import eu.kanade.tachiyomi.source.model.SChapter
 import eu.kanade.tachiyomi.source.model.SManga
-import java.io.Serializable
 import rx.Observable
 import uy.kohesive.injekt.Injekt
 import uy.kohesive.injekt.api.get
@@ -13,7 +12,7 @@ import uy.kohesive.injekt.api.get
 /**
  * A basic interface for creating a source. It could be an online source, a local source, etc...
  */
-interface Source : Serializable {
+interface Source {
 
     /**
      * Id for the source. Must be unique.