rtc: s3c: remove unnecessary NULL assignment

It's unnecessary the code that assigns info->rtc_clk to NULL in
s3c_rtc_remove.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>