wrapFirefoxAddonsHook: remove legacy toplevel-is-XPI code path

pkgs/firefox-extensions/wrapFirefoxAddonsHook/wrap-firefox-addons-hook.sh

@@ -50,16 +50,10 @@ fixupFirefoxAddon() {
 }
 
 fixupFirefoxAddons() {
-  if [ -f "$out" ]; then
-    # the output is a single firefox extension (legacy)
-    # TODO: remove this code path
-    fixupFirefoxAddon "$out"
-  else
   # crawl the output for .xpi files to fixup
   for xpi in $(@find@ "$out" -name '*.xpi'); do
     fixupFirefoxAddon "$xpi"
   done
-  fi
 }
 
 if [ -z "${dontFixupFirefoxAddons-}" ]; then