Can't delete empty partition
Solved. I just had to boot into recovery and run

diskutil cs unlockVolume C6735B8F-XXXX-XXXX-XXXX-XXXXXXXXXXXX
diskutil cs resizeStack C6735B8F-XXXX-XXXX-XXXX-XXXXXXXXXXXX 0g


The only strange thing is that Disk Utility shows a blue and white rectangle to represent the drive (unless this is simply showing used space):



Thanks goes to this post: Resizing or expanding a CoreStorage volume