[SearchResumption] Remove search resumption from testing configs.

The search resumption module is still shown sometimes on NTP due to the
feature flag is enabled in fieldtrial_testing_config.json. Remove the
entry.

Change-Id: I78f566272e691914d6559f4d0e648d6d39115357
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5731555
Reviewed-by: Sky Malice <skym@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1331253}