Re: [w3c/payment-request] remove currencySystem member (#694)

@adrianhopebailie, wrote: 

> I can make up a currency today and have a community who are willing to trade in that currency online but I am prohibited from using PR until I get my currency on some registry and wait for browsers to add support for it?

I think this is a very important discussion that we need to have. @ianbjacobs, @adrianhopebailie, something to add to the f2f agenda? The question is, how do we keep users safe from bad actors in the cryptocurrency ecosystem, while also empowering good actors? 

Lately, due to high fraud, companies like Google and Facebook have moved to ban advertising of cryptocurrencies on their platforms (with Twitter possibly following suit). This is very concerning, obviously, so we are interested in ways of minimizing the risks of users being asked to pay or buy fraudulent cryptocurrencies.  

> I propose that for CR we make no mention of ISO 4217 and stick with the reference to https://www.ecma-international.org/ecma-402/4.0/index.html#sec-iswellformedcurrencycode which appears to allow any uppercase alpha code that is exactly 3 chars long and doesn't appear to compare the value to a set of allowed values.

That's pretty much where we end up after this PR - except descriptively references ISO4217 so a reader knows about the 3-letter format. There is no algorithmic check that the code must exist in "ISO 4217" (i.e., it's just the well-formedness check, as you point out). 

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