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