It is not the presence of a "C Drive" or operating system partition on a drive that makes it bootable or not. In the standard installation of Winodws 7, 8 or 10 (and maybe even before Windows 7, I don't remember) there is a separate partition that usually does not have a drive letter assigned and is not visible in file explorer that the computer boots from. This is called the system partition. The system partition contains the files that then loads the operating from the "C Drive" partition which is called the boot partition. Yes, I think Microsoft named them backwards, but that's the way it is.
For a legacy BIOS computer, the physical drive itself will be the MBR partition type, with either a FAT32 or NTFS separate System Reserved partition that contains the boot files which then loads the OS from the larger normally NTFS boot partition (C Drive). The system partition the computer boots from must be marked as active, and the NTFS partition the OS resides on won't be (you can only have 1 active partition per physical drive, and that is what BIOS looks for - the active partition).
For a UEFI computer, the physical drive itself will be the GPT partition type and will have a separate FAT32 EFI System Partition which will then load the OS from the boot partition (C Drive). UEFI does not use the active partition marker - it looks for an EFI System Partition to boot from.
So - it's not really whether or not there is an OS installed on a partition on the drive that matters. It's whether or not there is an active partition containing the boots files on a legacy BIOS computer, or whether or not there is an EFI System partition on a UEFI computer. The user does not necessarily need to delete the C: drive partition - they need to remove the active partition (BIOS) or EFI System partition (UEFI). This is also the partition that must be cloned to the SSD to make it bootable.
Below, you can see that my volume 3 is the partition that the computer boots from, an EFI System partition, whereas volume 1 is the partition that contains the OS.
Microsoft Windows [Version 10.0.14393]
(c) 2016 Microsoft Corporation. All rights reserved.
Microsoft DiskPart version 10.0.14393.0
Copyright (C) 1999-2013 Microsoft Corporation.
On computer: JOHN-LAPTOP
DISKPART> list volume
Volume ### Ltr Label Fs Type Size Status Info
---------- --- ----------- ----- ---------- ------- --------- --------
Volume 0 G DVD-ROM 0 B No Media
Volume 1 C NTFS Partition 446 GB Healthy Boot
Volume 2 Recovery NTFS Partition 450 MB Healthy Hidden
Volume 3 FAT32 Partition 100 MB Healthy System
Volume 4 D Data NTFS Partition 856 GB Healthy
Volume 5 E NTFS Partition 69 GB Healthy
Volume 6 R RECOVERY FAT32 Partition 5633 MB Healthy
Volume 7 F Removable 0 B No Media