sched: Remove unneeed macro wrap

rt_entity_is_task has split definitions based on CONFIG_RT_GROUP_SCHED,
therefore we can use it always. No functional change intended.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250310170442.504716-3-mkoutny@suse.com