x86/mpx: Remove selftests themselves

MPX is being removed from the kernel due to a lack of support in the
toolchain going forward (gcc).

Remove the x86 selftests since they have been causing some issues because
of their propensity to do some debug-aiding tracepoint mucking.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190705175320.6542F8AE@viggo.jf.intel.com