**[CLN] payment(_*): correct code styling inconsistencies**

This commit mainly removes the extra indent level left over after commit odoo/odoo@8ec2e8cf807b3d3a8f84b8583887cb4b6a2828c1. It also cleans up purely cosmetic code styling inconsistencies.

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**[FIX] payment(_*): make a proper usage of async and promises**

Several functions were:
- called with `await` while there are synchronous;
- declared as synchronous while they should have been asynchronous;
- declared as synchronous but their overrides were async;
- explicitly encapsulating their return values in a `Promise` when it was unnecessary.

This commit also cleans up a few mistakes in comments and docstrings.

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**[REM] payment: remove unused payment method icons and SCSS rules**

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**[IMP] payment, \*: replace providers with payment methods in payment forms**

Before this commit, the payment providers (e.g., Stripe, Adyen...) available for payment were displayed on the payment forms. The customer had to select one to process their payment. After that, the customer had to select their preferred payment method (e.g., Credit Card, Bancontact...) from a list of payment methods supported by the selected provider over which the website administrator had close to no control. This was making the payment forms confusing because the payment methods were displayed sometimes more than once, if at all, in a non-controlled order, and behind the selection of a payment provider that customers should not have to deal with.

As the payment method was selected in an iframe or directly on the provider's website, the information on the selection payment method was not available in Odoo. This posed many problems, among which were the impossibility of assessing whether a specific feature (e.g., tokenization, refunds, manual capture...) was available, not being able to easily identify payment tokens through the payment method logo, listing available payment methods on the website, sorting and fine-grained configuration of the available payment method, subpar payment method-specific display on the payment form (e.g., PayPal that requires displaying a "Pay with PayPal" button), etc.

In this commit, the payment providers are thus replaced by the payment methods on the payment forms. All contextually available (depending on the country, currency, requested feature...) payment methods are displayed one after the other on a single-level list and in the order configured by the website administrator. Each payment method is "powered by" (i.e., linked) to a single payment provider: the first one, by model order, to support it. This allows, for example, offering the PayPal payment method through Mollie, which charges low processing fees, while also offering Klarna through Stripe, which supports more payment methods but charges higher processing fees.

While doing so, the two different payment forms, "Checkout" and "Manage", are also merged together in a new, configurable case-by-case, payment form that is entirely redesigned to offer a better user experience.

After payment, the information on the selected payment method is saved on the transaction and eventual payment record and updated with the information received from the provider.

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task-2882677

See also:
- https://github.com/odoo/enterprise/pull/40666
- https://github.com/odoo/upgrade/pull/5103
- https://github.com/odoo/documentation/pull/5717