Can't uninstall Boot Camp / restore to single partition

Some time ago, I attempted to install bootcamp and windows 10 on my iMac (late 2012) and botched it... partition is formed but boot camp and windows not loaded. I resorted to using parallels to run Windows and it works fine.

Now all I want to do is remove the bootcamp partition, but when I try using boot camp assistant, I get an error message "The startup disk cannot be partitioned or restored to a single partition - The startup disk must be formatted as a single Mac OS Extended (Journaled) volume or already partitioned by Boot Camp Assistant for installing Windows."

I used Boot Camp Assistant to set up the partition and the partition name is BOOTCAMP. How do I go about unifying my hard drive so I can take advantage of the 400 GB squandered in the BOOTCAMP partition?

Thank you.