hwspinlock/core: remove stubs for register/unregister

hwspinlock drivers must anyway select CONFIG_HWSPINLOCK,
so there's no point in having register/unregister stubs.

Removing those stubs will only make it easier for developers
to catch CONFIG_HWSPINLOCK mis-.config-urations.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>