sched: Convert CONFIG_RT_GROUP_SCHED macros to code conditions

Convert the blocks guarded by macros to regular code so that the RT
group code gets more compile validation. Reasoning is in
Documentation/process/coding-style.rst 21) Conditional Compilation.
With that, no functional change is expected.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250310170442.504716-2-mkoutny@suse.com