MIPS: Loongson32: Remove deprecated PWM timer clocksource

The Loongson1 PWM timer will be moved to clocksource framework.
Then, the old driver is no longer needed.
Remove the deprecated code and update the Kconfig.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230512103724.587760-2-keguang.zhang@gmail.com