sched: Remove sched_ktime_clock()

The timekeeping subsystem is suspended shortly before suspending
and resumed shortly after resuming. During this time, interrupts
are disabled and only one CPU is online. The scheduler shouldn't
really be involved at this point in suspend/resume, so having
this API doesn't really make any sense. Remove it.

Change-Id: Iad94c7a3b8f7cbad530d9ddbeb858d759119ab6e
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>