go/types: update internal (now incorrect) comment on deps field

The file ordering.go referred to in the comment was removed
with commit dd448957. There's no duplicate use of the deps
field anymore.

Change-Id: Ia6490e9f0839d4f755e8063758819e29b3d3b248
Reviewed-on: https://go-review.googlesource.com/106459


Reviewed-by: Matthew Dempsky <mdempsky@google.com>