niri: drop Steam Big Picture fullscreen rule

Big Picture reuses the existing client window (title change only), so the
open-fullscreen rule never fires on toggle; unused, removing it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

niri/config.kdl

@@ -184,16 +184,3 @@ window-rule {
         off
     }
 }
-
-// Steam Big Picture mode: open fullscreen, borderless
-window-rule {
-    match app-id="^steam$" title="^Steam Big Picture Mode$"
-    open-fullscreen true
-    open-floating false
-    border {
-        off
-    }
-    focus-ring {
-        off
-    }
-}