hw_breakpoint: Remove unused __register_perf_hw_breakpoint() declaration

Commit:

  b326e956 ("hw-breakpoints: Use overflow handler instead of the event callback")

removed __register_perf_hw_breakpoint() function usage and replaced it
with register_perf_hw_breakpoint() function.

Remove the left-over unused __register_perf_hw_breakpoint() declaration
from <linux/hw_breakpoint.h> as well.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/1594971060-14180-1-git-send-email-bhsharma@redhat.com