Question: I recently installed Ubuntu 12.10 and it requires a passphrase to boot up (I installed it with an encrypted file system).

Do I have to reinstall to change to a standard unencrypted file system?

Answer: 
If Ubuntu asks for an encryption passphrase during boot (i.e. on the text console before the login screen is displayed), this indicates that a full disk encryption method was used. (There's more than one way to do this, but I'll keep the answer general.) The encryption is handled by an extra software layer between the file system and the physical hard drive, not the file system itself.

There is no simple method or tool to undo this. With some knowledge about how Linux systems work, it can be done. You'd have to move the whole file system (or all files) to another partition (with enough free space) or external HDD. Then, remove the encrypted container, and recreate the file system without encryption. Finally, make sure that the new file system is properly recognized by the boot loader and `mount -a` before rebooting.

If possible, it's best to avoid this time consuming and error-prone procedure. Just do a fresh install. For a new user, this is the quickest and safest option.

PS: Chances are that you can change the encryption passphrase, possibly to an empty string. Then decrypting only requires to press Enter. Maybe you can go further and supress to (now useless) passphrase prompt. However, this does not disable the encryption. The data would still be encrypted although the encryption would be useless since the key can be trivially guessed.
It is actually possible to decrypt the partition in place without much effort. For example, see the instructions [here](https://wiki.archlinux.org/index.php/Removing_System_Encryption), which are generally just as valid for Ubuntu as for Arch. In my case, I had a LUKS1 device, which apparently makes things easier. All I had to do was this:

1. Boot into a live environment using a USB stick. I used Ubuntu 18.04.
2. Run `sudo cryptsetup-reencrypt --decrypt <device_path>`.

That was it. For a 250 GB SSD, it took 20 minutes. I didn't have to do anything special to `/etc/fstab`, grub, or initramfs. I commented out the relevant (only) line in `/etc/crypttab`, but I don't even think that was necessary.

That said, I had a second machine with a 500 GB SSD, and after about 3 hours, it still claimed it was going to be another 90 minutes and the rate was only getting slower, so I gave up and just reflashed the drive.

Before doing anything, though, I recommend backing up the (decrypted) partition. Using dd [as described in another answer](https://askubuntu.com/a/412737/173694) is great. I did it while the partition was mounted before I rebooted into the live USB environment. This came in handy for the partition I ended up reflashing, as I was able to remount the backup and copy over anything I cared strongly about.