Code: duplicate files

[timanovsky]

https://github.com/GoogleCloudPlatform/google-cloud-ruby/blob/master/google-cloud-core/lib/google/cloud/core/backoff.rb
and
https://github.com/GoogleCloudPlatform/google-cloud-ruby/blob/master/google-cloud-core/lib/google/cloud/core/grpc_backoff.rb

First should probably be deleted

[quartzmo]

Thanks for catching this duplication! Our goal is to replace this functionality soon with GAX, at which time we will remove both files.