Remove auth's AccountDecoder type

[alessio]

AccountDecoder is now entirely redundant.
client package now does no longer depend on x/auth.

Context: #4488

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added a relevant changelog entry: clog add [section] [stanza] [message]
• rereviewed Files changed in the github PR explorer

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[alexanderbez]

ACK