fix(registration): 🔥 remove polling logic

[lukas-runge]

fixes #31628

Notes: fix user registration resulting in firewall bans on homeservers (#31628)

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[t3chguy]

Polling is required for certain registration UIA flows, like e-mail verification, msisdn verification, terms acceptance, and likely any unspecified flows (given they can be extended with custom flows by the server) - how does this not prevent that from working? Tests would be required to assert no registration flows are broken for this to be removed.

[t3chguy]

Worth also noting the poll prop predates the community-contributed registration token support (by multiple years)