docs: Removed two images that were moved to Piper in April

Scott had migrated these graphics to Piper in April, but they've
stayed in Gerrit. These graphics had been updated in Gerrit since
then, so they got out of sync with Piper.

Set up Critique CL cr/133998971 to migrate the latest versions
to Piper (and sent to Scott for review); setting up a separate
CL here to remove the files, so they won't accidentally get
edited in the wrong place again.

Change-Id: Ice9dda71ae3604b1fc3aab15746dc41e759b80a6