Remove enbale_xpc_notifications compile flag

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index a4eb0660c9a45ed428d04351c8309d2c722f1476..7bce8b892e062ae127feba2d0c19b245178064b9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -517,11 +517,6 @@ group("gn_all") {
     deps += [
       "//breakpad:crash_inspector",
       "//breakpad:dump_syms",
-
-      # XPC service is in developement and guarded against the
-      # enable_xpc_notifications, in the meantime however we still
-      # need compile coverage in the bots.
-      "//chrome/browser/ui/cocoa/notifications:alert_notification_xpc_service",
       "//third_party/apple_sample_code",
       "//third_party/molokocacao",
     ]