rtc: simplified /proc/driver/rtc handling

This simplifies the RTC procfs support by removing the class_interface that
hooks it into the rtc core.  If it's configured, then sysfs support is now
part of the RTC core, and is never a separate module.

It also removes the class_interface hook, now that its last remaining user is
gone.  (That API is usable only with a "struct class_device".)

It's another step towards being able to remove "struct class_device".

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Acked-By: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>