NodeID: Remove unused BigInt format

[pav-kv]

This format was only used by HStar2 which was recently removed.

#2378

[codecov]

Codecov Report

Merging #2379 (4df6d3f) into master (3615e67) will decrease coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2379      +/-   ##
==========================================
- Coverage   66.05%   65.84%   -0.21%     
==========================================
  Files         107      106       -1     
  Lines        7834     7745      -89     
==========================================
- Hits         5175     5100      -75     
+ Misses       2117     2109       -8     
+ Partials      542      536       -6     

Impacted Files	Coverage Δ
merkle/smt/hstar3.go	90.90% <ø> (ø)
storage/tree/node.go	94.61% <ø> (+1.23%)	⬆️
log/operation_manager.go	87.30% <0.00%> (-1.06%)	⬇️
client/log_client.go	70.93% <0.00%> (-0.59%)	⬇️

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