Remove getFileType() and getChangeType() from DifferentialChangeset

Summary:
These are explicit copies of implicitly-generated Lisk methods.

See brief discussion in rPdec8bac3a3af6065166d485db80fffa70dc2abe3.

Test Plan: Looked at a diff in Differential.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2283