I can not find anywhere in the control panel where to delete a custom MX record so I can use Dreamhost's mail servers. Anyone know where or how to do this?
Is this what you are looking for? https://panel.dreamhost.com/index.cgi?tree=mail.mx&
Unfortunately, no. That page only allows you to ADD. You cannot REMOVE any custom MX records from that page.
Sure it does — if you want to stop using custom MX records and switch back to DreamHost mail, you just click the button under "Regular DreamHost Email".