boards: qemu_rx: remove zephyr-gnu toolchain

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

boards/qemu/rx/qemu_rx.yaml

@@ -10,7 +10,6 @@ arch: rx
 toolchain:
   - cross-compile
   - zephyr
-  - zephyr-gnu
 supported:
   - serial
 testing: