firestore: remove sort.Slice
firestore: remove sort.Slice

Forgot that it doesn't exist in go 1.6.

Change-Id: I46ffab39c09a06f8cb07688ca1d8bc39e3d24ab6
Reviewed-on: https://code-review.googlesource.com/25713
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