drivers: regulator: drop regulator_count_modes

The regulator_count_modes was an API that was not useful. Unlike with
voltages where the interface guarantees they are zero-indexed, modes can
take an arbitrary identifier. So counting supported modes doesn't
provide any useful information such as if a mode is allowed.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>