[Blink] Remove extra baseline file that was causing test failures

The virtual/omt-worker-fetch/external/wpt/xhr/
    xhr-timeout-longtask.any.worker.html test is consistently failing
on Mac with the following error:

"xhr-timeout-longtask.any.worker.html is a all-pass testharness test,
but has an extra baseline file. Please remove [the baseline file]."

Based on that message, this change removes the baseline file.

Bug: 1129910
Change-Id: Ia4462140d9efbc86b12cf679170bd6f5ee26e42a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418985Reviewed-by: Aleks Totic <atotic@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#808522}

third_party/blink/web_tests/platform/mac/virtual/omt-worker-fetch/external/wpt/xhr/xhr-timeout-longtask.any.worker-expected.txt

-This is a testharness.js-based test.
-FAIL Long tasks should not trigger load timeout assert_unreached: [object ProgressEvent] Reached unreachable code
-Harness: the test ran to completion.
-