sensor/nrf5_temp: Drop unncessary attribute set callback.

The sensor_attr_set() wrapper deals with drivers that do not support
the attribute set API, there is no need to provide a per driver
implementation that only return -ENOTSUP.

Change-Id: I1568ab018988fc560bd89ab96f2f2256ddd5b7e1
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>