x/build/builders: move 'commit' polling from the builder code to the coordinator

[bradfitz]

As part of gutting the builder and moving much of its logic to the coordinator (see
golang.org/s/builderplan), we should get rid of the commit polling. It's increasingly
not going to be used, since the builders won't be daemons.

[bradfitz]

Comment 1:

Labels changed: added builder.

[bradfitz]

Much of this has happened. But we do want to keep the old-style builder around, so best is probably just to update it to use thew new http://build.golang.org/?mode=json handler to find its own work, and remove the server's /todo handler entirely.