sched/core: Remove duplicate included header file stats.h

The header file stats.h is included twice. Remove the redundant include
and the following make includecheck warning:

  stats.h is included more than once

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250219111756.3070-2-thorsten.blum@linux.dev