ptpd_netif.h: remove prototypes and comment for unexisting code

The function are not there:

 * ptpd_netif_poll()
      it never existed

 * ptpd_netif_select()
      nobody called it, and it only returned 0

 * ptpd_netif_locking_enable()
 * ptpd_netif_locking_disable()
 * ptpd_netif_locking_poll()
 * ptpd_netif_adjust_counters()
 * ptpd_netif_adjust_phase()
 * ptpd_netif_adjust_in_progress()
 * ptpd_netif_enable_timing_output()
 * ptpd_netif_enable_phase_tracking()
 * ptpd_netif_read_calibration_data()
      they were part of ptp-noposix-wrappers.c, removed in January 2015.
      The features are now part of the wr_ops in ppsi.

 * ptpd_netif_get_ifName()
      same (ptp-noposix-wrappers.c), it returned a constant name, and
      ppsi does it differently because based on arch-wrs/arch-wrpc

 * ptpd_netif_calibrating_enable()
 * ptpd_netif_calibrating_disable()
 * ptpd_netif_extsrc_detection()
      same, but in addition they were already doing nothing when removed

 * ptpd_netif_calibrating_poll()
 * ptpd_netif_calibration_pattern_enable()
 * ptpd_netif_calibration_pattern_disable()
      same, but they wer not actually used since calibration was gone

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>