perf: Remove reference to AsyncTransferThread thread.

The 'AsyncTransferThread' was removed in crrev.com/345908. So remove the
reference to it from the benchmark code.

BUG=509076

Change-Id: I9b9fe755a87ea55eeff5ccc1c3a59943684a0958
Reviewed-on: https://chromium-review.googlesource.com/997102Reviewed-by: David Reveman <reveman@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548490}

tools/perf/metrics/timeline.py

@@ -16,7 +16,6 @@ from telemetry.web_perf.metrics import timeline_based_metric
 TimelineThreadCategories = {
     "Chrome_InProcGpuThread": "GPU",
     "CrGpuMain": "GPU",
-    "AsyncTransferThread": "GPU_transfer",
     "VizCompositorThread": "display_compositor",
     "CrBrowserMain": "browser",
     "Browser Compositor": "browser",