Fix rtutcp initialization for pymodbus 3.x (remove deprecated method kwarg)

[richardapeters]

Summary

Fixes RTU-over-TCP setup failure caused by passing method="rtu" to AsyncModbusSerialClient, which is not accepted in newer pymodbus versions.

Problem

Users configuring rtutcp hit:
AsyncModbusSerialClient.init() got an unexpected keyword argument 'method'

The integration currently sets:

• method: "rtu"
• port: "socket://:"

In current pymodbus APIs, AsyncModbusSerialClient already implies RTU framing, so method is unnecessary and invalid.

Change

In custom_components/deye_modbus/coordinator.py:

• Removed "method": "rtu" from params passed to AsyncModbusSerialClient.
• Kept port="socket://host:port" and serial params unchanged.

Why this works

AsyncModbusSerialClient determines RTU behavior by client type.
socket://... selects socket transport for serial-style RTU frames (RTU-over-TCP tunnel).

Impact

• Fixes startup/setup error for rtutcp on newer pymodbus.
• No behavior change for standard TCP path.