[gn build] Remove an unneeded -I flag

Everything includes clang/Config/config.h by qualified "clang/Config/config.h"
path, so there's no need for `-Igen/clang/include/clang/Config/clang/include`.

No behavior change.

llvm/utils/gn/secondary/clang/include/clang/Config/BUILD.gn

@@ -4,11 +4,6 @@ import("//llvm/utils/gn/build/libs/xml/enable.gni")
 import("//llvm/utils/gn/build/write_cmake_config.gni")
 import("//llvm/version.gni")
 
-config("Config_config") {
-  visibility = [ ":Config" ]
-  include_dirs = [ "$target_gen_dir/clang/include" ]
-}
-
 write_cmake_config("Config") {
   input = "config.h.cmake"
   output = "$target_gen_dir/config.h"
@@ -70,7 +65,4 @@ write_cmake_config("Config") {
   } else {
     values += [ "HOST_LINK_VERSION=" ]
   }
-
-  # Let targets depending on this find the generated file.
-  public_configs = [ ":Config_config" ]
 }