edac: core fix redundant sysfs controls to parameters

/sys/devices/system/edac/mc has a few files which are duplicated in
/sys/module/edac_core/parameters.  Now that all the functionality is
duplicated between these two locations, we remove the former kobject
attributes and update the documentation.

Signed-off-by: Arthur Jones <ajones@riverbed.com>
Signed-off-by: Doug Thompson <dougthompson@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>