Do not set SYSTEM_PRIORITY to system notifications

System notifications generated using CreateSystemNotification has
SYSTEM_PRIORITY. So we don't need to set SYSTEM_PRIORITY manually.

This CL is a clean-up and doesn't change any behavior.

Test: None
Bug: None
Change-Id: I8be104f87b756b5226f9034c9a31c67015978621
Reviewed-on: https://chromium-review.googlesource.com/906172
Reviewed-by: Mitsuru Oshima (In Tokyo) <oshima@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536651}
6 files changed