Centralise BeginTransaction and disable transaction use for now

osu.Game/Beatmaps/BeatmapManager.cs

@@ -184,7 +184,7 @@ namespace osu.Game.Beatmaps
             {
                 var context = importContext.Value;
 
-                using (var transaction = context.Database.BeginTransaction())
+                using (var transaction = context.BeginTransaction())
                 {
                     // create local stores so we can isolate and thread safely, and share a context/transaction.
                     var iFiles = new FileStore(() => context, storage);
@@ -295,7 +295,7 @@ namespace osu.Game.Beatmaps
             {
                 var context = importContext.Value;
 
-                using (var transaction = context.Database.BeginTransaction())
+                using (var transaction = context.BeginTransaction())
                 {
                     context.ChangeTracker.AutoDetectChangesEnabled = false;
 

osu.Game/Database/OsuDbContext.cs

@@ -92,6 +92,12 @@ namespace osu.Game.Database
             modelBuilder.Entity<BeatmapInfo>().HasOne(b => b.BaseDifficulty);
         }
 
+        public IDbContextTransaction BeginTransaction()
+        {
+            // return Database.BeginTransaction();
+            return null;
+        }
+
         public new int SaveChanges(IDbContextTransaction transaction = null)
         {
             var ret = base.SaveChanges();

osu.Game/Input/KeyBindingStore.cs

@@ -39,7 +39,7 @@ namespace osu.Game.Input
         private void insertDefaults(IEnumerable<KeyBinding> defaults, int? rulesetId = null, int? variant = null)
         {
             using (var context = GetContext())
-            using (var transaction = context.Database.BeginTransaction())
+            using (var transaction = context.BeginTransaction())
             {
                 // compare counts in database vs defaults
                 foreach (var group in defaults.GroupBy(k => k.Action))