Remove superfluous environment variables in test configurations.

The read barrier configuration is enabled by default, so we can remove
all environment definitions setting `ART_USE_READ_BARRIER` to `true`.

Test: art/test/testrunner/run_build_test_target.py art-test
Test: art/test/testrunner/run_build_test_target.py art-ndebug
Test: art/test/testrunner/run_build_test_target.py art-interpreter
Test: art/test/testrunner/run_build_test_target.py art-interpreter-access-checks
Test: art/test/testrunner/run_build_test_target.py art-jit
Test: art/test/testrunner/run_build_test_target.py art-pictest
Test: art/test/testrunner/run_build_test_target.py art-gcstress-gcverify
Test: art/test/testrunner/run_build_test_target.py art-read-barrier
Test: art/test/testrunner/run_build_test_target.py art-read-barrier-gcstress
Test: art/test/testrunner/run_build_test_target.py art-read-barrier-table-lookup
Test: art/test/testrunner/run_build_test_target.py art-tracing
Test: art/test/testrunner/run_build_test_target.py art-interpreter-tracing
Test: art/test/testrunner/run_build_test_target.py art-forcecopy
Test: art/test/testrunner/run_build_test_target.py art-no-prebuild
Test: art/test/testrunner/run_build_test_target.py art-no-image
Test: art/test/testrunner/run_build_test_target.py art-interpreter-no-image
Test: art/test/testrunner/run_build_test_target.py art-relocate-no-patchoat
Test: art/test/testrunner/run_build_test_target.py art-no-dex2oat
Test: art/test/testrunner/run_build_test_target.py art-preopt
Test: art/test/testrunner/run_build_test_target.py art-gtest
Test: art/test/testrunner/run_build_test_target.py art-gtest-read-barrier
Test: art/test/testrunner/run_build_test_target.py art-gtest-read-barrier-table-lookup
Bug: 62611253
Change-Id: I9926943631927b2056aa51ebaeb3ace263d50c1c