Arm backend: Remove deprecated Ethos-U registration shim (pytorch#22822)

Remove the registration shim deprecated in ExecuTorch 1.2. The backend
is registered automatically when linked.

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

Author: Sebastian-Larsson

backends/arm/runtime/EthosUBackend.cpp

@@ -466,15 +466,6 @@ Backend EthosUBackend_id{"EthosUBackend", &EthosUBackend_backend};
 static executorch::runtime::Error EthosUBackend_registered =
     register_backend(EthosUBackend_id);
 
-// DEPRECATED in Executorch 1.2
-// Remove it from your code and make sure to add this to your CMAKE rules
-// instead:
-//   executorch_target_link_options_shared_lib(executorch_delegate_ethos_u)
-extern "C" ET_DEPRECATED executorch::runtime::Error
-executorch_delegate_EthosUBackend_registered() {
-  return EthosUBackend_registered;
-}
-
 } // namespace
 
 } // namespace arm