Remove signature validation bypass in auth and backend

[mkurapov]

With the new http-signature-utils package (#665 #797), that enables signing requests locally, we can remove the BYPASS_SIGNATURE_VALIDATION flag and it's corresponding config.bypassSignatureValidation checks in the auth and backend packages.

[wilsonianb]

Removing this would allow you to run the backend on its own via

rafiki/packages/backend/src/open_payments/auth/middleware.ts

Lines 31 to 37 in b361754

	if (
	process.env.NODE_ENV !== 'production' &&
	token === config.devAccessToken
	) {
	await next()
	return
	}

but it would no longer be possible to run a standalone auth