x86/mm: Drop deprecated DISCONTIGMEM support for 32-bit

The DISCONTIGMEM support was marked as deprecated in v5.2 and since there
were no complaints about it for almost 5 releases it can be completely
removed.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20200223094322.15206-1-rppt@kernel.org