firestore: remove unused test.pb.go
firestore: remove unused test.pb.go

Change-Id: I0f700f8ea513aa05702cccb1b840c6152586fb3c
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/47891
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Chris Cotter <b879ae7d4b52f11ce9f60d9854075eb8eebaf7a9@google.com>

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