drivers/rtc/rtc-tps6586x.c: remove incorrect use of rtc_device_unregister

Device managed functions do not need explicit freeing/unregistering.
Moreover in this case it was done using non-device managed function
which is incorrect.  Hence remove it.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Laxman dewangan <ldewangan@nvidia.com>
Cc: Chiwoong Byun <woong.byun@samsung.com>
Cc: Jonghwa Lee <jonghwa3.lee@samsung.com>
Cc: Venu Byravarasu <vbyravarasu@nvidia.com>
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>