Remove ENABLE_VR_SHELL compile-time define.

chrome/browser/android/vr_shell/BUILD.gn

Index: chrome/browser/android/vr_shell/BUILD.gn
diff --git a/chrome/browser/android/vr_shell/BUILD.gn b/chrome/browser/android/vr_shell/BUILD.gn
index 067601f802221959029c48458c489d8250b57f6f..b5573b77e4de63ab9cf122e75adc91b1d659dfc7 100644
--- a/chrome/browser/android/vr_shell/BUILD.gn
+++ b/chrome/browser/android/vr_shell/BUILD.gn
@@ -6,7 +6,7 @@ import("//build/config/android/rules.gni")
 import("//chrome/common/features.gni")
 import("//testing/test.gni")
 
-assert(enable_vr_shell || enable_webvr)
+assert(enable_webvr)
 
 if (current_cpu == "arm" || current_cpu == "arm64") {
   static_library("vr_common") {
@@ -54,11 +54,7 @@ if (current_cpu == "arm" || current_cpu == "arm64") {
       "vr_web_contents_observer.h",
     ]
 
-    if (enable_vr_shell) {
-      defines += [ "ENABLE_VR_SHELL" ]
-    }
     if (enable_vr_shell_ui_dev) {
-      assert(enable_vr_shell)
       defines += [ "ENABLE_VR_SHELL_UI_DEV" ]
     }