How to remove Fax cover page?

No you can remove that.

See the exact similar requirement here: https://stackoverflow.com/questions/50993997/ringcentral-fax-coverpages/57509134#57509134

Setting coverIndex = 0 to disable cover page.
This parameter is actually optional in the API.
As per API reference here: "If not specified, the default cover page which is configured in 'Outbound Fax Settings' is attached"

https://developers.ringcentral.com/api-reference/Fax/createFaxMessage

As per this link, "When using these demos, the fax API coverIndex parameter should be set to 0 so that no cover page is added."

https://github.com/grokify/ringcentral-demos-fax-cover-page#ringcentral-fax-api-configuration-parameters