Remove line-related stats from Differential

Summary:
Ref T2825. Line stats are misleading when multiple commits are associated with a revision. It's better to have no data than misleading/bad data.

We can compute them accurately against commits via Fact/ETL at some point far in the future (see T1562 / T1135).

Test Plan: {F37519}

Reviewers: ptarjan

Reviewed By: ptarjan

CC: aran

Maniphest Tasks: T2825

Differential Revision: https://secure.phabricator.com/D5432