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As WriteOnRequestCompleted is always called whether the push operation
is cancelled successfully or not, this CL removes the unused cancelled
callback.

Bug: 1151857
Change-Id: I5f5bb163e52afe2f9b163a043fa25eb5def9cc8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2554607
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Rijubrata Bhaumik <rijubrata.bhaumik@intel.com>
Cr-Commit-Position: refs/heads/master@{#832308}