x86/nmi: Drop unused declaration of proc_nmi_enabled()

The declaration is unused as the definition got deleted.

Fixes: 5f2b0ba4 ("x86, nmi_watchdog: Remove the old nmi_watchdog").
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240306-const-sysctl-prep-x86-v1-1-f9d1fa38dd2b@weissschuh.net