gui: Remove unused `Decrypt` wallet

[maflcko]

The Decrypt case is unused and should be removed. See

src/qt/askpassphrasedialog.h:        Decrypt     /**< Ask passphrase and decrypt wallet */

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[sehyunc]

Hey MarcoFalke, I'm interested in working on this issue.

[hebasto]

@sehyunc

I'm interested in working on this issue.

Thanks! Will review your PR :)

[michaelfolkson]

Have you made any progress on this @sehyunc?

[sehyunc]

Have you made any progress on this @sehyunc?

Hello, yes, I've spent some time familiarizing myself with the code base and working on this issue.

[hebasto]

@MarcoFalke Mind looking into #109 ?