Remove the GetLeavesByIndex method from the log storage interface.

[pphaneuf]

Part of #2245.

[codecov]

Codecov Report

Merging #2426 (c7ae35a) into master (ab61c60) will increase coverage by 0.57%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2426      +/-   ##
==========================================
+ Coverage   65.17%   65.74%   +0.57%     
==========================================
  Files         104      104              
  Lines        7412     7299     -113     
==========================================
- Hits         4831     4799      -32     
+ Misses       2063     1990      -73     
+ Partials      518      510       -8     

Impacted Files	Coverage Δ
storage/cloudspanner/log_storage.go	55.76% <ø> (+5.85%)	⬆️
storage/memory/log_storage.go	3.61% <ø> (+0.10%)	⬆️
storage/mysql/log_storage.go	66.12% <ø> (+0.64%)	⬆️
storage/tools/dump_tree/dumplib.go	41.35% <0.00%> (ø)
log/operation_manager.go	87.30% <0.00%> (-2.12%)	⬇️

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