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/dex2oat/dex2oat_test.cc b/dex2oat/dex2oat_test.cc
index c076b5a..58dd047 100644
--- a/dex2oat/dex2oat_test.cc
+++ b/dex2oat/dex2oat_test.cc
@@ -396,6 +396,11 @@
 };
 
 TEST_F(Dex2oatSwapUseTest, CheckSwapUsage) {
+  // The `native_alloc_2_ >= native_alloc_1_` assertion below may not
+  // hold true on some x86 systems when read barriers are enabled;
+  // disable this test while we investigate (b/29259363).
+  TEST_DISABLED_FOR_READ_BARRIER_ON_X86();
+
   RunTest(false /* use_fd */,
           false /* expect_use */);
   GrabResult1();