panel-layout: drop a leftover declaration

1 job from master in 3 minutes and 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #104810
build-ubuntu

00:03:59

 
Name Stage Failure
failed
build-ubuntu Build
make[4]: Leaving directory '/builds/GNOME/gnome-panel/gnome-panel'
make[3]: *** [Makefile:1533: all-recursive] Error 1
make[3]: Leaving directory '/builds/GNOME/gnome-panel/gnome-panel'
make[2]: *** [Makefile:744: all] Error 2
make[2]: Leaving directory '/builds/GNOME/gnome-panel/gnome-panel'
make[1]: Leaving directory '/builds/GNOME/gnome-panel'
make[1]: *** [Makefile:540: all-recursive] Error 1
make: *** [Makefile:472: all] Error 2
ERROR: Job failed: exit code 1