Remove extra defines from GN build.

No longer needed with updated ANGLE/Chromium integration.

See http://anglebug.com/3320 for more context.
diff --git a/BUILD.gn b/BUILD.gn
index 5acbfff..cfd68b8 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -51,7 +51,7 @@
     "loader/generated",
     "loader",
   ]
-  defines = [ "API_NAME=\"Vulkan\"" ] + vulkan_loader_extra_defines
+  defines = [ "API_NAME=\"Vulkan\"" ]
 
   if (is_win) {
     cflags = [ "/wd4201" ]
diff --git a/build-gn/secondary/build_overrides/vulkan_loader.gni b/build-gn/secondary/build_overrides/vulkan_loader.gni
index 4b248e2..c826fc1 100644
--- a/build-gn/secondary/build_overrides/vulkan_loader.gni
+++ b/build-gn/secondary/build_overrides/vulkan_loader.gni
@@ -19,6 +19,5 @@
 vulkan_gen_subdir = "" 
 
 # Vulkan loader build options
-vulkan_loader_extra_defines = []
 vulkan_loader_shared = true