y2038: remove unused time interfaces

After many small patches, at least some of the deprecated interfaces
have no remaining users any more and can be removed:

  current_kernel_time
  do_settimeofday
  get_monotonic_boottime
  get_monotonic_boottime64
  get_monotonic_coarse
  get_monotonic_coarse64
  getrawmonotonic64
  ktime_get_real_ts
  timekeeping_clocktai
  timespec_trunc
  timespec_valid_strict
  time_to_tm

For many of the remaining time functions, we are missing one or
two patches that failed to make it into 4.19, they will be removed
in the following merge window.

The replacement functions for the removed interfaces are documented in
Documentation/core-api/timekeeping.rst.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>