cmake: nrf51: Don't #define NRF51822

The define NRF51822 has no in-tree users and is believed to have been
added by accident. The define NRF51 should be referenced in it's
place.

To avoid users accidentally referencing NRF51822 instead of NRF51, and
to avoid confusing users into believing that NRF51822 has semantics we
remove it.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>