sched/debug: Remove CONFIG_SCHED_DEBUG

For more than a decade, CONFIG_SCHED_DEBUG=y has been enabled
in all the major Linux distributions:

   /boot/config-6.11.0-19-generic:CONFIG_SCHED_DEBUG=y

The reason is that while originally CONFIG_SCHED_DEBUG started
out as a debugging feature, over the years (decades ...) it has
grown various bits of statistics, instrumentation and
control knobs that are useful for sysadmin and general software
development purposes as well.

But within the kernel we still pretend that there's a choice,
and sometimes code that is seemingly 'debug only' creates overhead
that should be optimized in reality.

So make it all official and make CONFIG_SCHED_DEBUG unconditional.

Now that all uses of CONFIG_SCHED_DEBUG are removed from
the code by previous patches, remove the Kconfig option as well.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250317104257.3496611-6-mingo@kernel.org