samples: modbus: rtu_server: drop unused variable
samples: modbus: rtu_server: drop unused variable

This has been unreferenced since:

4ff616b647 modbus: rework interface configuration

This generates a compiler warning, but it went unnoticed because the
sample test is configured incorrectly and not running.

Signed-off-by: Fabio Baltieri <161d407f4dc681bfd4aedb33b8273b2aafd727bc@gmail.com>

galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr