Remove JSON marshaler for base account

[alexanderbez]

Remove custom JSON marshaling for BaseAccount as multsigs cannot be bech32 decoded.

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• Code follows the module structure standards.
• Wrote unit and integration tests
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• Added relevant godoc comments.
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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
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[codecov]

Codecov Report

Merging #6054 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6054   +/-   ##
=======================================
  Coverage   54.69%   54.69%           
=======================================
  Files         424      424           
  Lines       25814    25790   -24     
=======================================
- Hits        14119    14107   -12     
+ Misses      10717    10708    -9     
+ Partials      978      975    -3     

[assafmo]

@alexanderbez Thanks for the swift response! 🎸