cmd/go: do not support code.google.com/r/zzz projects

This reverts CL 13261048. I have just learned that these are
no longer supported on code.google.com (that is, it is impossible
to create them), so there is little reason to add support in
Go 1.2.

Update issue 5408

R=golang-dev, dave, r
CC=golang-dev
https://codereview.appspot.com/13317046