Bimsebasse is mostly
A default installation of Mint includes GRUB 2, whether or not you're using another OS; it's just that this default installation of GRUB hides the GRUB menu at boot time (at least on a Mint-only installation), so that you don't realize it's installed. When you updated GRUB with the Windows 7 disk plugged in, the installation scripts detected Windows and added it to the menu. They may also have made the menu visible by default, and it's conceivable they overwrote the Windows boot loader on the external disk. The GRUB upgrade should not have deleted Windows itself, or even written to its partition.
Where bimsebasse isn't quite correct is that it is
possible to boot Linux (including Mint) without GRUB. To do so, you must install another boot loader. On BIOS-based computers, the usual choices for this are LILO, GRUB Legacy, GRUB 2, and SYSLINUX. GRUB 2 is currently the most popular of these by far, but my impression is that LILO (the oldest of the four) is making a modest resurgence, and some people are moving to SYSLINUX. On EFI-based computers, boot loader choices include ELILO, GRUB Legacy, GRUB 2, and the kernel's EFI stub loader (in kernels 3.3.0 and later). Of these, IMO GRUB 2 is the worst choice and the EFI stub loader is the most reliable. The EFI stub loader is also the newest, though, and in many configurations, you'll need a flexible boot manager to give you a menu of options. My EFI Boot Loaders for Linux
page gives more details on the EFI boot loaders. I don't know of a site offhand that gives a detailed comparison of the BIOS boot loaders.
If you really want to get rid of GRUB, you can do so, but to get more advice, you should post information on your firmware (BIOS vs. EFI) and your specific needs.