Can you delete it using Disk Utility?
I would not attempt this UNLESS you have a just-done backup of your main partition.
IF DU won't do it, my standard recommendation:
1. Use a cloning app (CarbonCopyCloner or SuperDuper) to clone the contents of your main partition to an external drive, creating a BOOTABLE backup.
2. BOOT FROM the cloned backup
3. Open Disk Utility and ERASE the internal fusion drive. Set it up with one partition only.
4. Now re-open your cloning app and RE-clone the backup BACK TO the internal drive.
This will take some time, but it -WILL- "do the job" properly.