clean up redundant static functions
convert `csl_rx_time` to uint32_t and remove redundant `nrf5_schedule_rx`
`nrf5_config_csl_period` and `nrf5_receive_at` static functions.
`csl_rx_time` has been passed as uint32. It is compared to uint64 value
returned by `nrf_802154_sl_timer_current_time_get` in
`nrf_802154_ie_writer.c` which leads to wrong calculation of phase in CSL.
Signed-off-by: Maciej Baczmanski <maciej.baczmanski@nordicsemi.no>
