KVM: selftests: Remove x86's so called "MMIO warning" test

KVM: selftests: Remove x86's so called "MMIO warning" test

Remove x86's mmio_warning_test, as it is unnecessarily complex (there's no
reason to fork, spawn threads, initialize srand(), etc..), unnecessarily
restrictive (triggering triple fault is not unique to Intel CPUs without
unrestricted guest), and provides no meaningful coverage beyond what
basic fuzzing can achieve (running a vCPU with garbage is fuzzing's bread
and butter).

That the test has *all* of the above flaws is not coincidental, as the
code was copy+pasted almost verbatim from the syzkaller reproducer that
originally found the KVM bug (which has long since been fixed).

Cc: Michal Luczaj <mhal@rbox.co>
Link: https://groups.google.com/g/syzkaller/c/lHfau8E3SOE
Link: https://lore.kernel.org/r/20230815220030.560372-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>