sponsors: remove airbrake

current sponsors are listed here (admin only):
https://github.com/sponsors/neovim/dashboard/your_sponsors?tier_id=41555

Author: justinmk

images/sponsors/airbrake-small.png

@@ -52,11 +52,6 @@ <h2>Sponsoring Neovim development</h2>
 
   <div>
     <h2>Sponsors</h2>
-    <div id="sponsors2">
-      <a href="https://airbrake.io/?utm_medium=sponsor&utm_source=Neovim&utm_content=airbrake-home-page&utm_campaign=2021-sponsorships">
-        <img width="300" height="102" src="/images/sponsors/airbrake-small.png" alt="Airbrake"/>
-      </a>
-    </div>
     <div id="sponsors3">
       <a href="https://rizin.re/">
         <img width="297" height="100" src="/images/sponsors/rizin.svg" alt="Rizin"/>