Remove entity lump manipulation from OnLevelInit

Newer Source engine versions now use a dynamically allocated buffer for
the map entity lump, and some maps have over 16MB of entity data - far
larger than our 2MB limit.

There is no sane way we can currently handle this, so just remove the
functionality from the forward until a more comprehensive API can be
designed.

Fixes #1470

plugins/include/sdkhooks.inc

@@ -379,8 +379,8 @@ forward Action OnGetGameDescription(char gameDesc[64]);
  * When the level is initialized
  *
  * @param mapName       Name of the map
- * @param mapEntities   Entities of the map
- * @return              Plugin_Changed if mapEntities has been edited, else no change.
+ * @param mapEntities   Unused, always empty
+ * @return              Unused, return value is ignored
  */
 forward Action OnLevelInit(const char[] mapName, char mapEntities[2097152]);