NUCLEO_L4R5ZI: remove IAR exporter
NUCLEO_L4R5ZI: remove IAR exporter

The STM32L4R5ZI device has been added in IAR V8.20.1

Remove the IAR exporter until IAR V8 is supported on mbed.

mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,c1728p9/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,andcor02/mbed-os