Revert of Support server categories in NTPSnippetsService. (patchset #3 id:40001 of https://codereview.chromium.org/2279863002/ )

Reason for revert:
I'm afraid this failed again on
https://build.chromium.org/p/chromium.mac/builders/Mac10.9%20Tests/builds/27385/steps/components_unittests%20on%20Mac-10.9

Original issue's description:
> Support server categories in NTPSnippetsService.
>
> Reland of:
> https://crrev.com/330958ddcdcd0084d724571eddaa66f9c364115f
>
> Not clear why tests failed; they passed for me and in the CQ.
> Removed one synchronization bit that wasn't really part of the change.
>
> BUG=633613
>
> Committed: https://crrev.com/880bb766ee4985a8dd36be10098c5196c41f7509
> Cr-Commit-Position: refs/heads/master@{#414681}

TBR=treib@chromium.org,sfiera@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=633613

Review-Url: https://codereview.chromium.org/2283743002
Cr-Commit-Position: refs/heads/master@{#414686}
4 files changed