Remove MPX leftovers in generic code.

Fixes: 45fc24e8 ("x86/mpx: remove MPX from arch/x86")
Signed-off-by: NJimmy Assarsson <jimmyassarsson@gmail.com>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Acked-by: NDave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20200402172507.2786-1-jimmyassarsson@gmail.com