clocksource/drivers/rda: drop redundant Kconfig dependency

Since commit 2f8a26c1

 ("clocksource: Improve GENERIC_CLOCKEVENTS
dependency") all clocksource drivers depend on GENERIC_CLOCKEVENTS so
drop the redundant attribute from the RDA-timer entry which was added
later.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200513122548.16974-1-johan@kernel.org