Remove demo payment method code

[nerrad]

This was introduced as a part of #1349 to provide a reference for how the payment method registration api works and to verify display etc. Once #1473 and #1471 are done, this code can be removed (and might be removed as a part of those pulls).

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[nerrad]

The payment demo folder will still exist, it just will be repurposed to be a built bundle for the payment method registration. So this issue can be closed.