samples: bmm150: Remove assignment to missing CONFIG_BMM150_SET_ATTR

This symbol never existed, and has some dead code associated with it:
https://github.com/zephyrproject-rtos/zephyr/issues/7848



Just remove the assignment to the undefined Kconfig symbol for now. The
associated code could be removed separately if it isn't needed anymore.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>