remove nerpanet related code

[jennijuju]

as the nerpa net was deprecated on aug 16 https://docs.filecoin.io/about-filecoin/networks/#deprecated-networks - removing related code from lotus

[codecov]

Codecov Report

Merging #7373 (f57d4b4) into master (fe56995) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7373      +/-   ##
==========================================
- Coverage   39.17%   39.15%   -0.02%     
==========================================
  Files         614      614              
  Lines       65005    65003       -2     
==========================================
- Hits        25465    25453      -12     
- Misses      35130    35146      +16     
+ Partials     4410     4404       -6     

Impacted Files	Coverage Δ
build/version.go	27.27% <ø> (+2.27%)	⬆️
chain/events/observer.go	71.64% <0.00%> (-6.72%)	⬇️
storage/wdpost_sched.go	75.24% <0.00%> (-5.95%)	⬇️
chain/store/store.go	63.66% <0.00%> (-2.12%)	⬇️
chain/stmgr/searchwait.go	62.09% <0.00%> (-1.31%)	⬇️
chain/vm/vm.go	59.47% <0.00%> (-1.12%)	⬇️
storage/wdpost_changehandler.go	97.64% <0.00%> (-0.95%)	⬇️
node/impl/full/chain.go	12.34% <0.00%> (-0.61%)	⬇️
chain/actors/builtin/miner/v5.go	53.53% <0.00%> (-0.31%)	⬇️
chain/stmgr/call.go	71.51% <0.00%> (ø)
... and 7 more

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[magik6k]

Should also remove build/params_nerpanet.go

[arajasek]

and build/genesis/nerpanet.car