boards: arm: nrf: remove note about board name change

Nordic Dev Kit board names were changed in Zephyr
v2.3 release, following the standard Board deprecation
policy. Two releases later we do not need to keep
references to the old names in the boards' documentation.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>