JANITORIAL: Remove useless headers

System headers are not allowed in engines code and are not used

engines/ags/shared/ac/game_setup_struct_base.cpp

@@ -26,7 +26,6 @@
 #include "ags/shared/ac/words_dictionary.h"
 #include "ags/shared/script/cc_script.h"
 #include "ags/shared/util/stream.h"
-#include <string.h>
 
 namespace AGS3 {
 

engines/groovie/logic/cake.cpp

@@ -25,7 +25,6 @@
  *
  */
 
-#include <limits.h>
 #include "groovie/groovie.h"
 #include "groovie/logic/cake.h"
 

engines/sky/compact.cpp

@@ -27,7 +27,6 @@
 #include "common/translation.h"
 #include "sky/compact.h"
 #include "gui/message.h"
-#include <stddef.h>	// for ptrdiff_t
 
 namespace Sky {