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: c96f57b0 ("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.comSigned-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/x86_64/amx_test.c

@@ -221,7 +221,7 @@ int main(int argc, char *argv[])
 	vm_vaddr_t amx_cfg, tiledata, xstate;
 	struct ucall uc;
 	u32 amx_offset;
-	int stage, ret;
+	int ret;
 
 	/*
 	 * Note, all off-by-default features must be enabled before anything
@@ -263,7 +263,7 @@ int main(int argc, char *argv[])
 	memset(addr_gva2hva(vm, xstate), 0, PAGE_SIZE * DIV_ROUND_UP(XSAVE_SIZE, PAGE_SIZE));
 	vcpu_args_set(vcpu, 3, amx_cfg, tiledata, xstate);
 
-	for (stage = 1; ; stage++) {
+	for (;;) {
 		vcpu_run(vcpu);
 		TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_IO);