TF pending internal changes no longer converted to PRs

[Austin Anderson]

Hi all,

FYI, I am reverting the change that creates a new PR for every pending internal TensorFlow changelist. That process caused development delays and confusing behavior for both internal and external developers; much more than I expected. This means that the explosion of PRs from copybara-service@ has stopped.

I'll still be working on higher quality presubmit checks for PRs, but the process will not be as streamlined as I had hoped.

Thank you,
Austin
TF DevInfra