[dev] Verify bailing out early in gateways is_available methods

[kalessil]

We do some consolidation for payment gateways is_available to bail out early based on 'yes' !== $this->enabled check, hence filing this ticket.

I believe the performance impact is low, but it should be verified or dismissed by those more familiar with this gateway.

For context, Woo core's \WC_Payment_Gateways::get_available_payment_gateways runs during checkout and checks each gateway's availability. We aim to maintain performance with disabled gateways as well.