Remove "(UNREGISTRED)" string from window title of Sublime Text 2

https://www.sublimetext.com/buy

Sublime Text may be downloaded and evaluated for free, however a license must be purchased for continued use.

Licenses are per-user, rather than per-machine, so you can enjoy Sublime Text on as many computers and operating systems as you wish with your license

Alternative Payment Methods

PayPal is currently the only way to purchase a license for Sublime Text. However, for most countries a PayPal account is not required; you can purchase as a guest using a credit card. Unfortunately, PayPal requires users in some countries, such as New Zealand, to create an account.

PayPal sometimes rejects valid credit cards. If this occurs, trying again later or using an alternate credit card has helped many customers.

Getting rid of the "unregistered" text will cost you 70 USD.

This is not software from Ubuntu itself. This is third party software and the creator of this software is free to choose their license. In this case it costs money.

Go to Preferences -> Settings - User and add

"update_check": false

to the file. This is made to disable automatic updates, but it also appears to disable the notification, at least for me.

While purchasing a legitimate copy is highly recommended, it is still the user's choice and not particularly a topic for this question.