sched/fair: Remove CONFIG_CFS_BANDWIDTH=n definition of cfs_bandwidth_used()

Andy reported that clang gets upset with CONFIG_CFS_BANDWIDTH=n:

  kernel/sched/fair.c:6580:20: error: unused function 'cfs_bandwidth_used' [-Werror,-Wunused-function]
   6580 | static inline bool cfs_bandwidth_used(void)
	|                    ^~~~~~~~~~~~~~~~~~

Indeed, cfs_bandwidth_used() is only used within functions defined under
CONFIG_CFS_BANDWIDTH=y. Remove its CONFIG_CFS_BANDWIDTH=n declaration &
definition.

Reported-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20241127165501.160004-1-vschneid@redhat.com