regulator: pwm-regulator: Remove obsoleted property

In "[3d7ef30] regulator: pwm-regulator: Simplify voltage to duty-cycle
call" we stopped using max_duty_cycle, so we can retire it from device
data and DT.

There is no need to deprecate this property, as it hasn't hit Mainline
yet.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>