Delete docs/upload.txt

"This document describes how the current uploading system is build up and how
custom backends can be built. (At least someday it will)."

It says this since 2009 (r55604). I see no need to keep that red herring here.

Change-Id: I70d20bf21527bee26bf244381a9d75d96856dd52
Merge "Delete docs/upload.txt"