drivers: clock_control: nrf: fix temperature sensor usage

The temperature sensor was only needed when
CONFIG_CLOCK_CONTROL_NRF_CALIBRATION_MAX_SKIP > 0. Implementation did
not reflect this dependency correctly, and sensor sampling code was
always compiled. Also removed CONFIG_MULTITHREADING checks, since this
driver is only compiled if CONFIG_MULTITHREADING=y.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>