spanner/internal/testutil: delete FuncMock
spanner/internal/testutil: delete FuncMock

Nothing uses it.

Change-Id: Ie35d55d558c52550f76375877f799417c925fd8a
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/43012
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