Attempting to revert to Mac OS X Mavericks, now can't get out of OS X Utilities!
Restart your Mac to Recovery Mode by pressing cmd R at startup
Launch Terminal from the menubar Utilities->Terminal
Enter the following command at the Terminal prompt:

defaults write com.apple.DiskUtility DUDebugMenuEnabled 1

Press enter
Quit Terminal
Start Disk Utility and enable Show every Partition in the Debug menu
After a few seconds all partitions should be visible
Activate the partition Macintosh HD if it's present and greyed out.
Erasing/formating of Macintosh HD should be possible now.


If this doesn't work quit Disk Utility and according to this site


Start Terminal and enter following command at the Terminal prompt: 

diskutil cs list


This will give you a list of the CoreStorage volumes on your system.   Your old FileVault 2-encrypted drive should be the only one listed.
Copy the Logical Volume Group (LVG) alphanumeric UUID of your CoreStorage volume. The LVG should be the first UUID listed and it’s the one you want to delete.
Next, run the following command:

diskutil cs delete lvgUUID #replace lvgUUID here by the UUID you found above


This will delete your CoreStorage volume and reformat it as an unencrypted HFS+ volume.


If it's still impossible to erase Macintosh HD or if it's not present consider the Internet Recovery Mode or a USB Recovery drive and format the whole internal 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 to the recovery netboot image which is loaded from an Apple/Akamai server.  
In the opening window choose Disk Utility and format/partition the internal drive. Choose Mac OS Extended (Journaled) and GUID partition table (available with the Options... button). Quit Disk Utility.
Plug in your Time Machine backup drive.
Start Time Machine and recover your old system