ramdisk: remove long-deprecated "ramdisk=" boot-time parameter

The "ramdisk" parameter was removed from the defunct rd.c file quite some
time ago, in favour of the more specific "ramdisk_size" parameter so, for
consistency, the same should be done here.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>