Remove OmniboxCombinedSpeculativeServiceWorker field trial testing config.

The "OmniboxCombinedSpeculativeServiceWorker" study has long expired and
the "OmniboxSpeculativeServiceWorkerStartOnQueryInput" feature has
been enabled by default in r541744.

Bug: 732681
Change-Id: I08175813d67ed834eba31c080e5071248fb4a49e
Reviewed-on: https://chromium-review.googlesource.com/954891
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541917}