Revert: Generalize auth/types.StdSignature (#4507)

[alexanderbez]

This reverts commit a32d5a4. I think we should first discuss what it is exactly we want to abstract out in terms of x/auth & it's dependencies.

A signature is nothing more than just a []byte and an accompanying crypto.PubKey which is already an interface. There is no need to further abstract this out. It's also not worth the amino storage overhead.

@ValarDragon suggested we have an Authenticator interface which is just a wrapper for VerifyBytes.

/cc @alessio @rigelrozanski

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[codecov]

Codecov Report

Merging #4512 into master will decrease coverage by 0.03%.
The diff coverage is 52.17%.

@@            Coverage Diff            @@
##           master   #4512      +/-   ##
=========================================
- Coverage   53.23%   53.2%   -0.04%     
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  Files         259     259              
  Lines       16206   16204       -2     
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- Hits         8628    8621       -7     
- Misses       6932    6937       +5     
  Partials      646     646