sched/fair: Remove unused cfs_rq_clock_task() function

cfs_rq_clock_task() was first introduced and used in:

  f1b17280 ("sched: Maintain runnable averages across throttled periods")

Over time its use has been graduately removed by the following commits:

  d31b1a66 ("sched/fair: Factorize PELT update")
  23127296 ("sched/fair: Update scale invariance of PELT")

Today, there is no single user left, so it can be safely removed.

Found via the -Wunused-function build warning.

Signed-off-by: Qian Cai <cai@lca.pw>
Cc: Ben Segall <bsegall@google.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/1568668775-2127-1-git-send-email-cai@lca.pw


[ Rewrote the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>