Remove unused Gson singleton factory

We don't inject it anywhere anymore.

(cherry picked from commit b47face2f8)

app/src/main/java/eu/kanade/tachiyomi/AppModule.kt

@@ -2,7 +2,6 @@ package eu.kanade.tachiyomi
 
 import android.app.Application
 import android.os.Handler
-import com.google.gson.Gson
 import eu.kanade.tachiyomi.data.cache.ChapterCache
 import eu.kanade.tachiyomi.data.cache.CoverCache
 import eu.kanade.tachiyomi.data.database.DatabaseHelper
@@ -44,8 +43,6 @@ class AppModule(val app: Application) : InjektModule {
 
         addSingletonFactory { TrackManager(app) }
 
-        addSingletonFactory { Gson() }
-
         addSingletonFactory { Json { ignoreUnknownKeys = true } }
 
         // SY -->