Boot Camp will not allow removing Windows 8 partition
Maybe there is a way to get this working but I and some technicians haven't found it. BootCamp got me hard headaches some times.

The fastest way would be to make a backup of your Macintosh HD (create a disk image) and partition the whole drive. Delete all partitions, apply, then create a new one and apply. Be sure to set the partition type to GUID.