The discount module status is not really defined. It has been abandoned several years ago. Try to find the latest version in the comments of the module maybe you will have more luck...

I just deleted the module folder after disabling it. Rather than run 'update db' I'd rather just manually delete the tables. Does discounts use its own tables, or were there shared tables? I see three tables with 'uc_discount' in the name, but I don't wanna drop them just yet.

You can exactly see what tables discount module use by checking the SQL request in the .install file.

If you want to make a clean uninstall, delete manually these tables.

But the best way to proceed is to use the uninstall tab in the module admin page, but in your case, it's too late. So do it manually

Helpful to know this module is not being actively developed. It seems to have issues with Ubercart 1.0 RC2, so I wouldn't advise using it. The Discounts module doesn't seem to support the Uninstall feature but here are the three tables that need to be dropped:
uc_discounts
uc_discounts_conditions
uc_discounts_actions

Code in the uc_discounts.install creates these three tables, and also included a uc_discounts_uninstall function which attempts to drop these three tables, so they are clearly safe to drop. I guess the uninstall code is buggy/out of date, since it doesn't seem to work in current releases.

The original issue could be due to not having one of the included discount modules installed. These modules provide the conditions and actions so if none are installed those boxes will be empty. I think we need a kind of 'requirements check' that does a drupal_set_message on the settings page when no conditions or actions are available.

I have just posted an issue for this: http://drupal.org/node/323868

I also just tested the uninstall procedure and it did drop the tables as expected.