netxduo: Remove workaround for THREADX_TOOLCHAIN and THREADX_ARCH

The autobot has updated NetX Duo to 2973652d which brings in `gnu` and
`ac6` ports for Cortex-M55 and Cortext-M85 with CMake support. There is
no more need to fall back to the port for Cortex-M0.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>