Server EntitySystem cleanup (#1617)

* Server EntitySystem cleanup

I went after low-hanging fruit systems.

* Add / change to internal access modifiers to systems
* Use EntityQuery to get components instead
* Add sealed modifier to systems
* Remove unused imports
* Add jetbrains annotation for unused classes
* Removed some pragmas for dependencies

This should also fix a decent chunk of the server build warnings, at least the ones that matter.

* Also disposals

* Update Content.Server/GameObjects/EntitySystems/GravitySystem.cs

* Fix build

Co-authored-by: Metal Gear Sloth <metalgearsloth@gmail.com>
Co-authored-by: Víctor Aguilera Puerto <6766154+Zumorica@users.noreply.github.com>

Content.Server/GameObjects/EntitySystems/ProjectileSystem.cs

@@ -1,6 +1,5 @@
 using Content.Server.GameObjects.Components.Projectiles;
 using JetBrains.Annotations;
-using Robust.Shared.GameObjects;
 using Robust.Shared.GameObjects.Systems;
 
 namespace Content.Server.Interfaces.GameObjects.Components.Interaction
@@ -8,25 +7,17 @@ namespace Content.Server.Interfaces.GameObjects.Components.Interaction
     [UsedImplicitly]
     internal sealed class ProjectileSystem : EntitySystem
     {
-        public override void Initialize()
-        {
-            base.Initialize();
-
-            EntityQuery = new TypeEntityQuery(typeof(ProjectileComponent));
-        }
-
         public override void Update(float frameTime)
         {
             base.Update(frameTime);
 
-            foreach (var entity in RelevantEntities)
+            foreach (var component in ComponentManager.EntityQuery<ProjectileComponent>())
             {
-                var component = entity.GetComponent<ProjectileComponent>();
                 component.TimeLeft -= frameTime;
 
                 if (component.TimeLeft <= 0)
                 {
-                    entity.Delete();
+                    component.Owner.Delete();
                 }
             }
         }