go: Disable additional signature verification for migration

[peterjgilbert]

Some places were missed when signature verification was disabled for the
purpose of migration.

[codecov]

Codecov Report

Merging #2656 into stable/20.3.x will decrease coverage by 0.05%.
The diff coverage is 31.57%.

@@                Coverage Diff                @@
##           stable/20.3.x    #2656      +/-   ##
=================================================
- Coverage          63.07%   63.02%   -0.06%     
=================================================
  Files                365      365              
  Lines              34386    34396      +10     
=================================================
- Hits               21690    21678      -12     
- Misses             10024    10039      +15     
- Partials            2672     2679       +7

Impacted Files	Coverage Δ
go/oasis-node/cmd/debug/fixgenesis/fixgenesis.go	8.53% <0%> (ø)	⬆️
go/consensus/tendermint/seed.go	70.65% <16.66%> (-2.08%)	⬇️
go/consensus/tendermint/tendermint.go	66.89% <37.5%> (-0.14%)	⬇️
go/registry/api/api.go	36.82% <50%> (ø)	⬆️
go/common/grpc/auth/auth.go	88.88% <0%> (-5.56%)	⬇️
go/worker/storage/service_external.go	47.19% <0%> (-4.5%)	⬇️
go/storage/mkvs/urkel/writelog/iterator.go	92.72% <0%> (-3.64%)	⬇️
go/runtime/history/prune.go	74.19% <0%> (-3.23%)	⬇️
go/worker/common/host/protocol/protocol.go	65.67% <0%> (-2.99%)	⬇️
go/storage/mkvs/urkel/db/api/helpers.go	64.78% <0%> (-2.82%)	⬇️
... and 14 more

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[pro-wh]

🎵 transfer