xrefs.Decorations: remove dynamic target_definitions computation
xrefs.Decorations: remove dynamic target_definitions computation

Summary: Use the serving data as the complete source-of-truth for the target definitions in a file.

Reviewers: #core_team, fromberger

Reviewed By: #core_team, fromberger

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1384

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