The nvram sysfs attributes have been deprecated at least since v4.13, more
than 3 years ago and nobody ever complained about the deprecation warning.

Remove the sysfs attributes now.

[Bartosz: remove the declaration of rtc_nvmem_unregister()]
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20201109163409.24301-5-brgl@bgdev.pl