spanner: remove duplicate header
spanner: remove duplicate header

We were setting the x-goog-api-client header twice.

Change-Id: Ia4576008dc9b33a5686c27c99ae9dbe0fcc139a4
Reviewed-on: https://code-review.googlesource.com/13150
Reviewed-by: Vikas Kedia <f6a34f50adc64ff8913356438469cce93f09537a@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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