Bug 1491536 - Remove obsolete footer image CSS. r=ntim

Differential Revision: https://phabricator.services.mozilla.com/D5960

--HG--
extra : moz-landing-system : lando

browser/base/content/browser.css

@@ -506,13 +506,6 @@ toolbar:not(#TabsToolbar) > #personal-bookmarks {
 }
 %endif
 
-:root[lwthemefooter=true] #browser-bottombox:-moz-lwtheme {
-  background-repeat: no-repeat;
-  background-position: bottom left;
-  background-color: var(--lwt-accent-color);
-  background-image: var(--lwt-footer-image);
-}
-
 /* Hide menu elements intended for keyboard access support */
 #main-menubar[openedwithkey=false] .show-only-for-keyboard {
   display: none;