remove popup for successful export

[sssoleileraaa]

Description

Towards freedomofpress/securedrop-client#556

[emkll]

Should we also remove for print as well? https://github.com/freedomofpress/securedrop-export/pull/19/files#diff-f7920fa15cd44ca4817dafa773cf5372L374

If so, we should also remove the method itself since it will no longer be used: https://github.com/freedomofpress/securedrop-export/pull/19/files#diff-f7920fa15cd44ca4817dafa773cf5372L141

[sssoleileraaa]

Ah, yes, the "Printing test page" popup should be part of the client messaging system when we add support for printing test pages to the client. It makes sense to remove unused code when we get rid of that, whether or not we do it in this PR

Also just a heads-up that this PR will remain open until freedomofpress/securedrop-client#556 is resolved.

[redshiftzero]

handled in #46, so closing this one