Revert "Add a frame generator that allows scrolling over a larger still image, for use with new screen sharing quality tests."

This reverts commit 89934133988feaf14fb88f5258f727232a283d0f.

BUG=
R=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1258803003 .

Cr-Commit-Position: refs/heads/master@{#9653}