Clean up JniMethodStart().

After we have clarified that @FastNative methods cannot be
synchronized and starting enforcing it in
    https://android-review.googlesource.com/1518545
we can clean up JniMethodStart() and remove the check for
@FastNative (though we keep a CHECK in debug builds).

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 172332525
Change-Id: I1466dbca3b22524b8cc72db79bf583d64d32c7bf
1 file changed