Fix paypal payment processor default configuration

Description

The `cancel_url` configuration setting is not used by the paypal payment
processor in ecommerce. Instead, the payment processor uses the
`cancel_checkout_path` configuration parameter.

This error has been reported by two different users:
https://discuss.overhang.io/t/ecommerce-plugin-500-error-on-fresh-setup
https://openedx.slack.com/archives/C02SNA1U4/p1573488822105900?thread_ts=1573488555.104900&cid=C02SNA1U4

The proposed fix has worked in both cases.

Assignee

Emma Green

Reporter

Open Source Pull Request Bot

Labels

Contributor Name

Régis Behmo

Repo

edx/ecommerce

Customer

None

Epic Link

None

OSCM Assignee

None

Platform Map Area (Levels 1 & 2)

None

Platform Map Area (Levels 3 & 4)

None

Blended Hour Utilization Percentage

None

Priority

Unset
Configure