sched: dynamic: Simplify irqentry_exit_cond_resched()

PREEMPT_DYNAMIC is now limited to the FULL and LAZY preemption models.
In either model, irqentry_exit_cond_resched() is always called and never
disabled.

Remove the unnecessary code for this when PREEMPT_DYNAMIC is selected.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>
Tested-by: Mete Durlu <meted@linux.ibm.com>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Tested-by: Jinjie Ruan <ruanjinjie@huawei.com>
Link: https://patch.msgid.link/20260803191731.3244294-5-mark.rutland@arm.com