drivers: s3c44b0_rtc: delete an unused driver

Since commit 5dc5f36 removed B2 board support,
there are no boards enabling s3c44b0_rtc.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Andrea Scian <andrea.scian@dave-tech.it>