Remove OnEntitySpawned C++ listener (#1342)

This is causing crashes with existing extensions using entity listeners.

Currently no one is asking for a C++ hook here, if it is desired it
needs to be done with proper versioning of the listener interface, which
is going to be a little bit more complicated without an initial hard
break.

https://forums.alliedmods.net/showpost.php?p=2715337&postcount=789
https://discordapp.com/channels/335290997317697536/335290997317697536/748101258186850334

This is a partial revert of #1078.

public/extensions/ISDKHooks.h

@@ -36,7 +36,7 @@
 #include <IShareSys.h>
 
 #define SMINTERFACE_SDKHOOKS_NAME		"ISDKHooks"
-#define SMINTERFACE_SDKHOOKS_VERSION	2
+#define SMINTERFACE_SDKHOOKS_VERSION	1
 
 class CBaseEntity;
 
@@ -71,16 +71,6 @@ namespace SourceMod
 		virtual void OnEntityDestroyed(CBaseEntity *pEntity)
 		{
 		}
-
-		/**
-		 * @brief	When an entity is spawned
-		 *
-		 * @param	pEntity		CBaseEntity entity.
-		 * @param	classname	Entity classname.
-		 */
-		virtual void OnEntitySpawned(CBaseEntity *pEntity, const char *classname)
-		{
-		}
 	};
 
 	/**