layers: Remove unused threadchecker header file

Change-Id: I7906b82c1c9c0397113508da74348ea28fe21add
diff --git a/layers/threading.cpp b/layers/threading.cpp
index 252a8c8..bb48a74 100644
--- a/layers/threading.cpp
+++ b/layers/threading.cpp
@@ -27,7 +27,6 @@
 #include "vk_layer_config.h"
 #include "vk_layer_extension_utils.h"
 #include "vk_layer_utils.h"
-#include "vk_enum_validate_helper.h"
 #include "vk_struct_validate_helper.h"
 #include "vk_layer_table.h"
 #include "vk_layer_logging.h"