clocksource/drivers/arc_timer: Remove duplicate error message

The function arc_get_timer_clk() prints an error message if it fails,
remove the second error message if the function fails.

Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200429151223.3120-1-zhengdejin5@gmail.com