KVM: selftests: Delete superfluous, unused "stage" variable in AMX test

KVM: selftests: Delete superfluous, unused "stage" variable in AMX test

Delete the AMX's tests "stage" counter, as the counter is no longer used,
which makes clang unhappy:

  x86_64/amx_test.c:224:6: error: variable 'stage' set but not used
          int stage, ret;
              ^
  1 error generated.

Note, "stage" was never really used, it just happened to be dumped out by
a (failed) assertion on run->exit_reason, i.e. the AMX test has no concept
of stages, the code was likely copy+pasted from a different test.

Fixes: c96f57b08012 ("KVM: selftests: Make vCPU exit reason test assertion common")
Reviewed-by: Jim Mattson <jmattson@google.com>
Link: https://lore.kernel.org/r/20240109220302.399296-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>