go: Re-enable signature verification disabled for migration

[Yawning]

Now that the migration has hopefully been done, re-enable all of the
signature verification that was disabled for the sake of allowing for
migration.

Fixes #2615

[codecov]

Codecov Report

Merging #2621 into master will increase coverage by 0.12%.
The diff coverage is 27.08%.

@@            Coverage Diff             @@
##           master    #2621      +/-   ##
==========================================
+ Coverage   62.82%   62.95%   +0.12%     
==========================================
  Files         365      365              
  Lines       34468    34424      -44     
==========================================
+ Hits        21653    21670      +17     
+ Misses      10139    10079      -60     
+ Partials     2676     2675       -1

Impacted Files	Coverage Δ
go/consensus/tendermint/apps/registry/genesis.go	53.04% <ø> (+0.9%)	⬆️
go/consensus/tendermint/tendermint.go	67.31% <0%> (+1.51%)	⬆️
go/oasis-node/cmd/debug/fixgenesis/fixgenesis.go	10.29% <0%> (+1.75%)	⬆️
go/registry/api/sanity_check.go	61.86% <0%> (+2.66%)	⬆️
go/consensus/tendermint/seed.go	72.72% <0%> (+2.07%)	⬆️
go/registry/api/api.go	36.8% <31.42%> (-0.02%)	⬇️
...nsus/tendermint/apps/supplementarysanity/checks.go	50.45% <33.33%> (-1.47%)	⬇️
go/control/debug.go	67.74% <0%> (-6.46%)	⬇️
go/runtime/history/prune.go	74.19% <0%> (-3.23%)	⬇️
go/consensus/tendermint/roothash/roothash.go	68.91% <0%> (-2.63%)	⬇️
... and 8 more

---

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[Yawning]

This should back out the horrific evil done in #2614, but it can't be merged till a release with said evil is made.