x86/setup: Remove unused MCA variables

Commit

  bb8187d3 ("MCA: delete all remaining traces of microchannel bus support.")

removed the remaining traces of Micro Channel Architecture support but
one trace remained - three variables in setup.c which have been unused
since 2012 at least.

Drop them finally.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201021165614.23023-1-bp@alien8.de