readme: remove waffle as not supported soon
readme: remove waffle as not supported soon

Fixes #10176

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