tests: kernel: tickless: Remove old board name

Removes a duplicated old board name

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

tests/kernel/tickless/tickless_concept/testcase.yaml

@@ -8,7 +8,6 @@ tests:
       - rv32m1_vega/openisa_rv32m1/zero_riscy
       - rv32m1_vega/openisa_rv32m1/ri5cy
       - nrf5340dk/nrf5340/cpunet
-      - nrf5340dk_nrf5340_cpunet
       - nucleo_l073rz
     tags:
       - kernel