powerpc: Remove dead non-preemption code

powerpc: Remove dead non-preemption code

Since commit 7dadeaa6e851 ("sched: Further restrict the preemption
modes"), powerpc always has CONFIG_PREEMPTION because only
CONFIG_PREEMPT and CONFIG_PREEMPT_LAZY are possible, even in
dynamic preemption mode (see sched_dynamic_mode).

As a consequence, need_irq_preemption() is always true and can be
removed.

And because commit bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY
feature") includes linux/irq-entry-common.h which already declares
sk_dynamic_irqentry_exit_cond_resched static key, asm/preempt.h
becauses useless and can be removed.

Signed-off-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/2bf10a0afffefb6aca44bf2f864cc17471a80e31.1781870889.git.chleroy@kernel.org