docs: proc: remove description of prof_cpu_mask

docs: proc: remove description of prof_cpu_mask

Commit 2e5449f4f21a ("profiling: Remove create_prof_cpu_mask().") said that
no one would create /proc/irq/prof_cpu_mask since commit 1f44a225777e
("s390: convert interrupt handling to use generic hardirq", 2013). Remove
the outdated description.

While at it, fix another minor typo (s/DMS/DMA/).

Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260311070940.94838-1-zenghui.yu@linux.dev>