drivers/block: remove CONFIG_EXPERIMENTAL

This config item has not carried much meaning for a while now and is
almost always enabled by default. As agreed during the Linux kernel
summit, remove it.

CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC: Asai Thambi S P <asamymuthupa@micron.com>
CC: Pete Zaitcev <zaitcev@redhat.com>
CC: Cong Wang <xiyou.wangcong@gmail.com>
CC: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>