drivers/rtc/rtc-ds1742.c: remove unused field "rtc" from private structure

Private field "rtc" is not used outside "probe", so there is no reason to
keep it.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Cc: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>