spanner: log failed to delete session in refreshIdle return true as the original session is successfully refreshed
spanner: log failed to delete session in refreshIdle
return true as the original session is successfully refreshed

Change-Id: Ib07a1bae9f2d86f65a8c55322302cdae5d61e24f
Reviewed-on: https://code-review.googlesource.com/13190
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Vikas Kedia <f6a34f50adc64ff8913356438469cce93f09537a@google.com>

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