gn: Stop checking for gyp-style build dirs.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index b9eb2ef50aae60463036b01f372b4d93ee90eaf9..0c2b660a3649df1335bc3ebef513590fac2d1289 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -13,14 +13,6 @@ import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build/toolchain/goma.gni")
 
-# One common error that happens is that GYP-generated headers within gen/ get
-# included rather than the GN-generated ones within gen/ subdirectories.
-# TODO(GYP_GONE): Remove once GYP is gone (as well as exec_script exception).
-assert(
-    exec_script("//build/dir_exists.py", [ "obj.host" ], "string") == "False",
-    "GYP artifacts detected in $root_build_dir.$0x0A" +
-        "You must wipe this directory before building with GN.")
-
 declare_args() {
   # When set (the default) enables C++ iterator debugging in debug builds.
   # Iterator debugging is always off in release builds (technically, this flag