Clear duplication of code in x/auth/client/cli/decode.go

[alessio]

x/auth/client/cli.GetDecodeCommand() and x/auth/client/cli.GetDecodeTxCmd() should be collapsed into one command as there's a clear duplication of code, names and functionalities.

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[girishramnani]

@alessio is this issue being actively worked on? if not I would like to work on it