firestore: remove ErrConcurrentTransaction
firestore: remove ErrConcurrentTransaction

It was never actually used anywhere.

Fixed doc as well.

Change-Id: Ic248b8cc723f01f5c95eaa1d7bb13367ed77df79
Reviewed-on: https://code-review.googlesource.com/30891
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go