Can't delete Logical Volume Group on Fusion drive
If all other means fail to remove a CoreStorage Volume Group you can always use gpt to remove the CS partition (or the CS partitions if it is a Fusion Drive).

Preparation:


Detach your backup drive
Restart to Internet Recovery Mode by pressing alt cmd R at startup. 

The prerequisites are the latest firmware update installed, either ethernet or WLAN (WPA/WPA2) and a router with DHCP activated.
On a 50 Mbps-line it takes about 4 min (presenting a small animated globe) to boot into a recovery netboot image which usually is loaded from an Apple/Akamai server.

I recommend ethernet because it's more reliable. If you are restricted to WIFI and the boot process fails, just restart your Mac until you succeed booting.

Alternatively you may start from a bootable installer thumb drive (preferably Yosemite or El Capitan) or a thumb drive containing a full system (preferably Yosemite or El Capitan). If you boot to a full system and login as admin you have to prepend sudo to execute some commands like gpt ... or newfs_hfs ...!


Modify the GUID partition table and replace your current LVG


Enter diskutil list and gpt -r show /dev/diskX (with diskX: the disk identifiers of your internal disks (probably disk0 SSD/disk1 HDD) to get an overview. In the commands below I assume the disk identifiers are disk0/disk1.
To delete a partition use: 

diskutil umountDisk /dev/disk2 #unmount the Logical Volume(s)
diskutil umountDisk /dev/disk0
diskutil umountDisk /dev/disk1
gpt remove -i index_number /dev/disk0
gpt remove -i index_number /dev/disk1


Usually the Physical Volumes of a Fusion Drive have the index number 2:

gpt remove -i 2 /dev/disk0
gpt remove -i 2 /dev/disk1

Re-add the partitions with the same start block and sizes as they had previously

gpt add -b StartBlock -i 2 -s size -t 48465300-0000-11AA-AA11-00306543ECAC /dev/disk0
gpt add -b StartBlock -i 2 -s size -t 48465300-0000-11AA-AA11-00306543ECAC /dev/disk1

Format the new partition:

newfs_hfs -v "SSD" -J /dev/disk0s2
newfs_hfs -v "HDD" -J /dev/disk1s2

Create a new Logical Volume Group (or "Fusion Drive"):

diskutil cs create name disk0s2 disk1s2 #name: name of the future LVG (e.g. FusinDrive)

Create a new Logical Volume:

diskutil cs list
diskutil cs createVolume lvgUUID jhfs+ name 100% #lvgUUID: the UUID of the Logical Volume Group, name: name of the future LV (e.g. MacintoshHD)


or

diskutil cs createVolume lvgUUID jhfs+ name size #size in t, g, m (e.g. 3100g)

To quit Terminal enter:

exit


and quit Terminal
Open Disk Utility to verify the new LVG/LV
Attach the backup drive
Open Time Machine and restore your backup