Remove enable_vulkan from build/config, and move it to the buildflag_header system.

No intended behavior change.

TBR=avi
BUG=none
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2744973002
Cr-Original-Commit-Position: refs/heads/master@{#456525}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7f118188c140989c8a633c60d531b4c08eef8564

config/ui.gni

@@ -35,9 +35,6 @@ declare_args() {
 
   # Whether we should use glib, a low level C utility library.
   use_glib = is_linux
-
-  # Enable experimental vulkan backend.
-  enable_vulkan = false
 }
 
 declare_args() {