edac: edac_mc no longer deals with kobjects directly

There are no more embedded kobjects in struct mem_ctl_info. Remove a header and
a comment that does not reflect the code anymore.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>