x86, boot: Remove obsolete and unused constant RAMDISK

The named constant RAMDISK is unused. It used to set the (obsolete)
kernel boot header field ram_size, but its usage for that purpose got
dropped in commit 5e47c478

 ("x86: remove
zImage support"). Now remove this constant too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Link: http://lkml.kernel.org/r/1345396003.1771.9.camel@x61.thuisdomein
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>