samples: Don't read board from cmake -DBOARD=x

The CMakeLists.txt file was reading ${BOARD}, but this means the user
must set BOARD like this cmake -DBOARD=foo. The user must be allowed
to set BOARD from the environment.

The code was unnecessary anyway because the convention
prj_${BOARD}.conf is known by boilerplate and therefore not necessary
to specify.

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