Cleanup gun containers (probably last time) (#8575)

- snake_case instead of kebab-case
- Remove old MagazineBarrel containers
- Remove old event

Content.Shared/Weapons/Ranged/Events/MagazineAutoEjectEvent.cs

@@ -1,13 +0,0 @@
-using Robust.Shared.Serialization;
-
-namespace Content.Shared.Weapons.Ranged.Events
-{
-    /// <summary>
-    /// This is sent if the MagazineBarrel AutoEjects the magazine
-    /// </summary>
-    [Serializable, NetSerializable]
-    public sealed class MagazineAutoEjectEvent : EntityEventArgs
-    {
-        public EntityUid Uid;
-    }
-}