Re-enable Stripe payouts transfer_group parameter in a while

[frankie567]

Historically, we were setting transfer_group on Pledge's payment intents. With our new payout procedure, this is not relevant anymore: transfer_group is set for the whole payout. However, this causes a bug when trying to payout payment intents: transfer group cannot be reset.

In 6cafff5, we remove transfer_group setting on Pledges. However, since there are some pledges with it in the wild, we had to temporarily disable transfer_group on payouts in 108d7e7.

In a few weeks, we should revert 108d7e7 so we correctly set transfer_group on payouts.