Revert "Respect assistant notification expiration"

This reverts commit 31b40abf6abe5caeef7cb41f3f612a802c2706c6.

Reason for revert: Causes build failure on Linux ChromiumOS MSAN builder:
https://ci.chromium.org/p/chromium/builders/ci/Linux%20ChromiumOS%20MSan%20Builder/14283?blamelist=1#blamelist-tab

Original change's description:
> Respect assistant notification expiration
> 
> Assistant notifications can now be given an expiration time through the
> |expiry_timestamp_ms| field.
> 
> A new AssistantNotificationExpiryMonitor class will monitor all
> notifications and remove them (from the AssistantNotificationModel)
> when they expire.
> 
> BUG=b/116618188
> TEST=unittests and manual create notification with expiry_timestamp_ms
> 
> Change-Id: I5d0c0b3bcd3a26d772bccb883a5a8d5914d9c929
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1688019
> Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Reviewed-by: Tao Wu <wutao@chromium.org>
> Commit-Queue: Jeroen Dhollander <jeroendh@google.com>
> Cr-Commit-Position: refs/heads/master@{#677864}

TBR=dcheng@chromium.org,xiaohuic@chromium.org,wutao@chromium.org,jeroendh@google.com

Change-Id: I7c2c13378691819e3e1741612f8bab5ad6c14eed
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: b/116618188
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1703097
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677873}
10 files changed