Linux 4.20: do_settimeofday is gone

Linux 4.20: do_settimeofday is gone

With Linux commit 976516404ff3fab2a8caa8bd6f5efc1437fed0b8 'y2038:
remove unused time interfaces', do_settimeofday() is gone.

However, OpenAFS only calls do_settimeofday() from afs_osi_SetTime(),
which has been dead code since -settime support was removed from afsd
with commit 1d9888be486198868983048eeffabdfef5afa94b 'Remove
-settime/RXAFS_GetTime client support'.

Instead of fixing afs_osi_SetTime() to use a current Linux API, remove
it as dead code.

No functional change is incurred by this commit.  However, this change
is required in order to build OpenAFS on Linux 4.20.

Reviewed-on: https://gerrit.openafs.org/13392
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit aa80f892ec39e2984818090a6bb2047430836ee2)

Change-Id: I7f5ee9c21bc87cd261d87126bc3848d941ff5409
Reviewed-on: https://gerrit.openafs.org/13406
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>