How to remove OS X Installer for 10.10. to get back to 10.7, the originally installed OS?
Something really, really bad happened to your internal 1 TB drive:

The GUID partition table was somehow replaced by a dual (and therefore additionally false) MBR partition table.

You may recover the previous GUID and your missing volume with a disk recovery tool like Disk Warrior to some extent. But since you have a backup just restore it:


In case you have a Time Machine backup:


Boot to Internet Recovery Mode by pressing altcmdR. The prerequisites for an Internet Recovery are listed here: Recovery/Internet Recovery.
In a few words you need the latest firmware update installed, either ethernet or WLAN (WPA/WPA2) and a router with DHCP activated for a successful internet recovery.
On a 50 Mbps-line it takes about 4 min (presenting a small animated globe) to boot into the Recovery Netboot image which is loaded from an Apple server.  

(Alternatively you may boot to an external drive containing a Recovery HD or a full OS X system or a OS X thumb drive.)
Open "Disk Utility" and partition your internal disk: GUID partition scheme (use the "Options..." button at the bottom of the main pane for this), one partition, journaled HFS+.  
Close "Disk Utility".  
Attach your Time Machine backup drive.  
Open "Restore from Time Machine Backup" and restore your main drive if you have booted from a Recovery drive.
If you've booted from a full OS X external drive just open Time Machine and restore your internal drive.

In case you have a different back-up app:


Follow the instructions of the non-Apple back-up application