Revert "gosrc: include package updated time in the request header."

The current process to fetch GitHub packages check the reference first,
which is the API call I use in the change for timestamp check. It turns
out the last modified timestamp returned can be older than the most
recent commit.

This reverts commit 3abf7616294048af14f0c5e99fc63ed7d8277bc7.

Change-Id: I55d38f55ee118ed6069dd9b3231858cebcf95281
Reviewed-on: https://go-review.googlesource.com/22700
Reviewed-by: Andrew Gerrand <adg@golang.org>
10 files changed