[w3c/payment-handler] Simplify Payment Handler Registration example. (#204)

It is unnecessary to call `navigator.permissions.query`. Instead
only the `PaymentManager.requestPermission` call is made. Also,
the feature check for `PaymentManager` is made earlier.

Fixes #200.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/payment-handler/pull/204

-- Commit Summary --

  * Simplify Payment Handler Registration example.

-- File Changes --

    M index.html (21)

-- Patch Links --

https://github.com/w3c/payment-handler/pull/204.patch
https://github.com/w3c/payment-handler/pull/204.diff

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