Remove the unused KeyHint field.

[pphaneuf]

Reviewing commit by commit might be easier.

Checklist

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

[codecov]

Codecov Report

Merging #2442 (71e498f) into master (6127136) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2442      +/-   ##
==========================================
- Coverage   66.15%   66.09%   -0.07%     
==========================================
  Files         104      104              
  Lines        7204     7191      -13     
==========================================
- Hits         4766     4753      -13     
  Misses       1924     1924              
  Partials      514      514              

Impacted Files	Coverage Δ
storage/cloudspanner/log_storage.go	56.03% <ø> (-0.09%)	⬇️
storage/mysql/log_storage.go	65.87% <ø> (-0.09%)	⬇️
types/logroot.go	100.00% <ø> (ø)
crypto/signer.go	86.95% <100.00%> (-1.05%)	⬇️
trees/trees.go	83.33% <100.00%> (ø)

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[pphaneuf]

This PR might be easiest to review commit-by-commit.