x86: Drop obsolete ARCH_BOOTMEM support

x86 unconditionally uses NO_BOOTMEM so there is no use
of the HAVE_ARCH_BOOTMEM support as mm/bootmem.c is the
only file referencing this symbol.

bootmem_arch_preferred_node() is the function referred
in the mm/bootmem.c code and can thuis be dropped too.

x86 was the sole user of HAVE_ARCH_BOOTMEM - so there is
an opportunity to clean up a little in mm/bootmem.c too
if we do not expect other users to emerge.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/20120406124735.GA6920@merkur.ravnborg.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>