xen: pcpu: remove unnecessary __ref annotation

The __ref annotation has been there since many years now, but the
reason why it has been added has been removed from the code long ago.

[jgross: clarify commit message]

Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: xen-devel@lists.xenproject.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sergio Miguéns Iglesias <sergio@lony.xyz>
Reviewed-by: Juergen Gross <jgross@suse.com>
Message-ID: <20241202085910.5539-1-sergio@lony.xyz>
Signed-off-by: Juergen Gross <jgross@suse.com>