Remove unused beta param for silu, use torch op directly

The beta param was only accepted on the tensorflow/torch backends
and not in the `keras.ops` API, nor was it tested. I think best
just to ditch, since no one could be relying on it.

keras/backend/tensorflow/nn.py

@@ -40,8 +40,8 @@ def softsign(x):
     return tf.nn.softsign(x)
 
 
-def silu(x, beta=1.0):
-    return tf.nn.silu(x, beta=beta)
+def silu(x):
+    return tf.nn.silu(x)
 
 
 def log_sigmoid(x):

keras/backend/torch/nn.py

@@ -47,9 +47,9 @@ def softsign(x):
     return tnn.softsign(x)
 
 
-def silu(x, beta=1.0):
+def silu(x):
     x = convert_to_tensor(x)
-    return x * sigmoid(beta * x)
+    return tnn.silu(x)
 
 
 def log_sigmoid(x):