Remove final traces of long-deprecated "ramdisk" kernel parm

Since the "ramdisk" kernel parameter has been officially deprecated
since at least 2.6.18, might as well finally get rid of it.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>