serving/xrefs: remove PagedCrossReferences.feature checks
serving/xrefs: remove PagedCrossReferences.feature checks

Summary: All feature checks are obsolete since the alternative slow paths were removed in D1932.

Reviewers: #core_team, shahms

Reviewed By: #core_team, shahms

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D2082

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