🔥 Drop an unused variable

I think the storage of binding data to `binding` is a hangover from some
version of this code; it isn't used so remove it.

Also ensure that the key that is returned is stripped of any surrounding
whitespace.

src/braindrop/app/commands/base.py

@@ -92,8 +92,8 @@ def key_binding(cls) -> str:
         if isinstance(key := cls.BINDING_KEY, tuple):
             key, *_ = key
         if isinstance(key, str):
-            key, _, _ = (binding := cls.binding()).key.partition(",")
-        return key or ""
+            key, _, _ = cls.binding().key.partition(",")
+        return (key or "").strip()
 
     @classmethod
     def action_name(cls) -> str: