Don't have my pfSense nearby, so I propose you what I should do (try):
If memory serves me well, the html code for these portal login pages are stored in the config file.
Export the config file - it's an XML file.
Edit it, empty the part that contains YOUR html code.
Don't touch anything else.
Import it back in pfSense.
The field will be empty, pfSEnse will propose the build-in pages, as shown above.
Btw: this was valid for 1.2.3 - I'm using 2.0 Release for 2 days now, and still under a shock. pfSense was amazing, now it became incredible ...