powercap/drivers/dtpm: Remove unused function definition

The dtpm.h header file is exporting a function which is not
implemented neither needed. Remove it.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20211123101601.2433340-1-daniel.lezcano@linaro.org

include/linux/dtpm.h

@@ -70,6 +70,4 @@ void dtpm_unregister(struct dtpm *dtpm);
 
 int dtpm_register(const char *name, struct dtpm *dtpm, struct dtpm *parent);
 
-int dtpm_register_cpu(struct dtpm *parent);
-
 #endif