policy: remove misleading comment in DeviceOffHoursProto

Remove misleading comment from DeviceOffHoursProto. The timezone
is not optional and must be valid.

Bug: b:166556614
Change-Id: I9f230297e8aeeb5861ef6834564d763bff6afeb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536436Reviewed-by: Sergey Poromov <poromov@chromium.org>
Commit-Queue: Victor-Gabriel Savu <vsavu@google.com>
Cr-Commit-Position: refs/heads/master@{#827235}

components/policy/proto/chrome_device_policy.proto

@@ -1292,7 +1292,6 @@ message DeviceOffHoursProto {
   // are in the timezone specified by the timezone string.
   repeated WeeklyTimeIntervalProto intervals = 1;
   // Timezone in the same format as SystemTimezoneProto.timezone.
-  // If timezone isn't set then "GMT" will be used
   optional string timezone = 2;
   // List of policy proto tags which settings are ignored during OffHours
   // mode. List contains policy proto tags from ChromeDeviceSettingsProto