Remove the incomplete postgres storage backend.

[pphaneuf]

We do not have the resources to maintain this storage backend (#1298).

Checklist

• I have updated the CHANGELOG.
• I have updated documentation accordingly (including the feature implementation matrix).

[codecov]

Codecov Report

Merging #2432 (77a6f1c) into master (09456fa) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2432      +/-   ##
==========================================
- Coverage   65.95%   65.92%   -0.03%     
==========================================
  Files         104      104              
  Lines        7240     7240              
==========================================
- Hits         4775     4773       -2     
- Misses       1951     1952       +1     
- Partials      514      515       +1     

Impacted Files	Coverage Δ
client/log_client.go	76.15% <0.00%> (-2.65%)	⬇️
log/operation_manager.go	88.35% <0.00%> (+1.05%)	⬆️

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