supporter: Remove bogus value for PayPal srt variable.

srt is meant to be used for time-limited subscriptions. 0 is not a valid
value. See
<https://developer.paypal.com/docs/classic/paypal-payments-standard/integration-guide/Appx_websitestandard_htmlvariables/#recurring-payment-variables>;.
It's optional, and leaving it unset gives us the behavior we want, so do
that instead.