rtc/alpha: remove legacy rtc driver

The old drivers/char/rtc.c driver was originally the implementation
for x86 PCs but got subsequently replaced by the rtc class driver
on all architectures except alpha.

Move alpha over to the portable driver and remove the old one
for good.

The CONFIG_JS_RTC option was only ever used on SPARC32 but
has not been available for many years, this was used to build
the same rtc driver with a different module name.

Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: linux-alpha@vger.kernel.org
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200226224322.187960-2-alexandre.belloni@bootlin.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>