CMake: Fix Microlib selection (#13871)
CMake: Fix Microlib selection (#13871)

As the assembler does not seem to recognise the request to the
linker to use microlib, remove that instruction.
Microlib is still used as the linker selects microlib as a result
of being explicitly asked to do so.
mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed