go.sum: purge unused dependencies
go.sum: purge unused dependencies

Change-Id: I0e75725696c4c5fe7bade91db6169398f7be0a1a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/166998
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

golang/protobuf,golang/protobuf