samples: drivers: mbox: remove mimxrt595_evk_cm33

Remove mimxrt595_evk_cm33 from the MBOX sample. Although this SOC has
support for both cores, there is no support in tree for booting the
secondary core from the primary one, so this sample cannot run on
hardware.

Fixes #69092

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>