remove the TrillianLog.GetLeavesByHash API

[pphaneuf]

Remove the TrillianLog.GetLeavesByHash API, as it is unused.

This is part of #2245.

Checklist

• I have updated the CHANGELOG.
  • Adjust the draft version number according to semantic versioning rules.
• I have updated documentation accordingly (including the feature implementation matrix).

[codecov]

Codecov Report

Merging #2243 (b5ab878) into master (9d9c7c9) will decrease coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2243      +/-   ##
==========================================
- Coverage   65.47%   65.29%   -0.18%     
==========================================
  Files         104      104              
  Lines        7511     7475      -36     
==========================================
- Hits         4918     4881      -37     
- Misses       2074     2075       +1     
  Partials      519      519              

Impacted Files	Coverage Δ
server/interceptor/interceptor.go	83.08% <ø> (+1.22%)	⬆️
server/log_rpc_server.go	82.95% <ø> (-1.12%)	⬇️
server/validate.go	93.33% <ø> (-0.52%)	⬇️
client/log_client.go	70.93% <0.00%> (-1.17%)	⬇️
log/operation_manager.go	87.30% <0.00%> (-1.06%)	⬇️

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[AlCutter]

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