Disable CheckSwapUsage test with read barriers on x86.

This test does not produce the expected results on
some x86 systems when read barriers are enabled.

Also remove unused TEST_DISABLED_FOR_READ_BARRIER macro
definition.

Test: m test-art-host-gtest
Bug: 29259363
Bug: 12687968
Change-Id: If7c124226b32c10425725e62bb684150aba108ab
diff --git a/runtime/common_runtime_test.h b/runtime/common_runtime_test.h
index b68eb19..8f7d18b 100644
--- a/runtime/common_runtime_test.h
+++ b/runtime/common_runtime_test.h
@@ -205,6 +205,12 @@
     return; \
   }
 
+#define TEST_DISABLED_FOR_READ_BARRIER_ON_X86() \
+  if (kUseReadBarrier && kRuntimeISA == kX86) { \
+    printf("WARNING: TEST DISABLED FOR READ BARRIER ON X86\n"); \
+    return; \
+  }
+
 }  // namespace art
 
 namespace std {