Can we remove our Custom SSL and switch to the SSL provided by Platform.sh?

Yes. Platform.sh issues a free Let's Encrypt SSL Certificate that is renewed automatically every 90 days. 

Retrieve the list of certificates with:

platform certificate:list

From that list, get the CERTIFICATE_ID of the certificate you want to delete.

You can remove your custom certificate with the following command:

platform certificate:delete <CERTIFICATE_ID>

You can then redeploy and the Let's Encrypt SSL Certificate will be automatically applied.

For more information on this feature, please see the following page in the Upsun documentation:

https://docs.platform.sh/define-routes/https.html#tls-certificates