go/ias-proxy: remove use genesis option

[ptrus]

This was used long ago, not needed anymore.

I guess it's technically a breaking change.

[codecov]

Codecov Report

Merging #4098 (79cad76) into master (2391dab) will decrease coverage by 0.59%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4098      +/-   ##
==========================================
- Coverage   67.13%   66.53%   -0.60%     
==========================================
  Files         411      410       -1     
  Lines       42320    42292      -28     
==========================================
- Hits        28412    28141     -271     
- Misses       9928    10165     +237     
- Partials     3980     3986       +6     

Impacted Files	Coverage Δ
go/oasis-node/cmd/ias/proxy.go	46.51% <ø> (-4.25%)	⬇️
...nsensus/tendermint/apps/keymanager/transactions.go	0.00% <0.00%> (-52.95%)	⬇️
go/ias/http/http.go	18.51% <0.00%> (-44.45%)	⬇️
go/keymanager/api/policy_sgx.go	23.07% <0.00%> (-15.39%)	⬇️
go/storage/api/context.go	86.36% <0.00%> (-13.64%)	⬇️
go/runtime/host/sandbox/sandbox.go	69.76% <0.00%> (-11.25%)	⬇️
go/storage/api/root_cache.go	75.55% <0.00%> (-8.89%)	⬇️
go/common/sgx/aesm/aesm.go	62.61% <0.00%> (-8.42%)	⬇️
...nsensus/tendermint/apps/beacon/state/state_pvss.go	69.23% <0.00%> (-7.70%)	⬇️
...consensus/tendermint/apps/keymanager/keymanager.go	57.22% <0.00%> (-6.63%)	⬇️
... and 39 more

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