Remove OnEntitySpawned forward (#1571)

`OnEntitySpawned` is 1.11 only, so this is fine given our API stability guarantees.

Unfortunately the forward name clashes with quite a few plugins using the same name for their SDKHook callback. Normally we'd just put up with this but there are difficult to solve binary compatibility issues where those plugins will get the callback double-called, and there is a separate issue where the forward isn't called for all entity spawns (unlike the SDKHook), so most plugins can't switch to the forward anyway.

Resolves #1558.

This reverts commit 7bab9cc344.

plugins/include/sdkhooks.inc

@@ -348,16 +348,6 @@ typeset SDKHookCB
  */
 forward void OnEntityCreated(int entity, const char[] classname);
 
-/**
- * When an entity is spawned
- *
- * @param entity        Entity index
- * @param classname     Class name
- *
- * @note Check for support at runtime using GetFeatureStatus on SDKHook_OnEntitySpawned capability.
- */
-forward void OnEntitySpawned(int entity, const char[] classname);
-
 /**
  * When an entity is destroyed
  *