Remove Cloud Spanner support.

[pphaneuf]

Part of #2245.

[codecov]

Codecov Report

Merging #2423 (e6b2e54) into master (eaad43e) will increase coverage by 3.96%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2423      +/-   ##
==========================================
+ Coverage   65.95%   69.92%   +3.96%     
==========================================
  Files         104       99       -5     
  Lines        7273     6094    -1179     
==========================================
- Hits         4797     4261     -536     
+ Misses       1966     1429     -537     
+ Partials      510      404     -106     

Impacted Files	Coverage Δ
log/operation_manager.go	88.35% <0.00%> (+1.05%)	⬆️

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[pav-kv]

I remember from @AlCutter that we run some internal CloudSpanner instance. Defer the review to Al.

[pphaneuf]

Sure thing. I think we've stopped using that instance, but should get confirmed, @AlCutter should know.

[AlCutter]

The CI/CD instance uses CloudSpanner

[pphaneuf]

Seems that we're using it for some post-submit testing, so we'll keep it.