Remove Legacy Migration Code

[alexanderbez]

Summary

There are two files that I believe are related to legacy migration paths/logic:

• app/migrate.go
• app/pubkey_replacement.go

These files relay on legacy SDK code that is marked deprecated and will be removed in future a release.

Not sure who the best person to ask is, but I wager we can just probably just remove these now.

/cc @yaruwangway @okwme

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[faddat]

Since they're in the history, I think it makes sense to thin out by removing them from the current main branch. Less to digest means more understanding for new readers.