Remove the unused GetLeavesByIndex API.

[pphaneuf]

Part of #2245.

[codecov]

Codecov Report

Merging #2425 (b5c2a41) into master (341f077) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2425      +/-   ##
==========================================
- Coverage   65.34%   65.20%   -0.14%     
==========================================
  Files         104      104              
  Lines        7453     7412      -41     
==========================================
- Hits         4870     4833      -37     
+ Misses       2064     2062       -2     
+ Partials      519      517       -2     

Impacted Files	Coverage Δ
client/log_client.go	76.58% <ø> (+2.33%)	⬆️
server/interceptor/interceptor.go	83.67% <ø> (-0.25%)	⬇️
server/log_rpc_server.go	80.55% <ø> (-1.74%)	⬇️
server/validate.go	92.72% <ø> (-0.61%)	⬇️

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