Remove DiffCommit.file_count and DiffSet.file_count fields

Review Request #10216 — Created Oct. 11, 2018 and submitted — Latest diff uploaded

Review Board
10233, 10232

This patch removes the above mentioned fields because we will now be
tracking per-commit FileDiffs and cumulative (i.e., a cumulative diff of
the entire commit series) FileDiffs in the DiffSet.files field, and
the file_count field will no longer be of any use in that case.

Since the mixin defining the file_count would only now be providing a
utility method, the mixin has been removed and the method has been moved
into a free function which both DiffCommit.get_total_line_counts and
DiffSet.get_total_line_counts call into.

Ran unit tests.