go/extra/stats: remove the unmaintained extra/stats tool

[ptrus]

The extra/stats tool is not used anymore and is not being kept up-to-date with oasis-core updates.

[codecov]

Codecov Report

Merging #3270 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3270      +/-   ##
==========================================
+ Coverage   67.70%   67.88%   +0.18%     
==========================================
  Files         371      371              
  Lines       36874    36874              
==========================================
+ Hits        24965    25032      +67     
+ Misses       8724     8682      -42     
+ Partials     3185     3160      -25     

Impacted Files	Coverage Δ
go/consensus/tendermint/api/errors.go	86.66% <0.00%> (-13.34%)	⬇️
go/common/grpc/errors.go	78.26% <0.00%> (-10.87%)	⬇️
go/consensus/tendermint/epochtime/epochtime.go	85.71% <0.00%> (-7.94%)	⬇️
go/storage/metrics.go	80.76% <0.00%> (-3.85%)	⬇️
go/storage/mkvs/tree.go	89.28% <0.00%> (-3.58%)	⬇️
go/consensus/tendermint/apps/staking/state/gas.go	81.03% <0.00%> (-3.45%)	⬇️
go/storage/api/root_cache.go	77.77% <0.00%> (-3.18%)	⬇️
go/keymanager/client/client.go	84.29% <0.00%> (-2.48%)	⬇️
go/worker/common/committee/node.go	75.48% <0.00%> (-2.41%)	⬇️
go/consensus/api/grpc.go	68.27% <0.00%> (-2.27%)	⬇️
... and 33 more

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