Arm backend: Remove stale broadcast helper docs (pytorch#19367)

Update the broadcast_tensors() docstring now that the old
reshape_for_broadcast helper has been removed.

Signed-off-by: Sebastian Larsson <sebastian.larsson@arm.com>

Author: Sebastian-Larsson

backends/arm/tosa/utils.py

@@ -64,16 +64,14 @@ def are_fake_tensors_broadcastable(
 def broadcast_tensors(tosa_fb, nodes: list[Node]) -> list[Any]:
     """Broadcast the FX nodes to a shared shape inside the TOSA graph.
 
-    This mirrors ``reshape_for_broadcast`` but also emits the tile operators
-    needed to materialize the broadcast and supports any number of inputs.
+    This emits the reshape and tile operators needed to materialize the
+    broadcast and supports any number of inputs.
 
     Args:
         tosa_fb (Any): TOSA graph builder that receives the broadcast
             operators.
         nodes (list[Node]): FX nodes whose tensor metadata should be
             broadcast.
-        tosa_spec (TosaSpecification): Active TOSA specification used to
-            decode tensor metadata.
 
     Returns:
         list[Any]: Broadcast versions of the inputs. Each element is either