remove CodeSponsor (#68)

see https://codesponsor.io/shutdown-notice/

Author: DanielRuf

README.md

@@ -16,10 +16,6 @@
 
 Mitt was made for the browser, but works in any JavaScript runtime. It has no dependencies and supports IE9+.
 
-<a target='_blank' rel='nofollow' href='https://app.codesponsor.io/link/WbARjbDRQz5y3N6VBEMPU4LW/developit/mitt'>
-  <img alt='Sponsor' width='888' height='68' src='https://app.codesponsor.io/embed/WbARjbDRQz5y3N6VBEMPU4LW/developit/mitt.svg' />
-</a>
-
 ## Table of Contents
 
 -   [Install](#install)