Jetpack: Subscriptions: Remove the shortcode

[scruffian]

The Subscriptions block outputs a shortcode.

Describe the solution you'd like

We should remove this, and output the markup we want for the block directly as suggested by @mtias

Describe alternatives you've considered

Keeping the shortcode means that people who use the shortcode continue to be able to benefit from the additional features we add, but this results in more complex development and doesn't encourage people to move to Gutenburg.

[mmtr]

We created a separated issue for stopping using the simple-payment shortcode as output of the "Pay with PayPal" block: https://github.com/Automattic/jetpack/issues/17246 (fixed by https://github.com/Automattic/jetpack/pull/17256).

I've updated this issue to reflect we only need to handle the Subscriptions block now.

[simison]

Thanks for this! Leaving the shortcode in Simple payments block was one of my biggest regrests. 😅

It came from wish to not add yet another simple payments implementation (alongside widget, classic editor block, and shortcode) but it turned out to be short-sighted and caused more trouble than it was aiming to save time.