Cleanup mimic event (#23705)

* Cleanup mimic event

Now it won't be forced on dev map or mapping mode.

* Minor cleanup

Content.Server/Antag/Mimic/MobReplacementRuleComponent.cs

@@ -0,0 +1,21 @@
+using Robust.Shared.Prototypes;
+
+namespace Content.Server.Antag.Mimic;
+
+/// <summary>
+/// Replaces the relevant entities with mobs when the game rule is started.
+/// </summary>
+[RegisterComponent]
+public sealed partial class MobReplacementRuleComponent : Component
+{
+    // If you want more components use generics, using a whitelist would probably kill the server iterating every single entity.
+
+    [DataField]
+    public EntProtoId Proto = "MobMimic";
+
+    /// <summary>
+    /// Chance per-entity.
+    /// </summary>
+    [DataField]
+    public float Chance = 0.001f;
+}