Remove DifferentialChangesetRenderer->(g|s)etLineCount()

Summary: The "line count" is always the same as count($this->getOldLines()), and somewhat misleading since it's really "number of rows in the two-up view". D4421 removed the only (or only remaining?) call.

Test Plan: looked at some diffs

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D4422