Revert "Multithreaded encoder, late sync loopfilter"
This commit is incomplete, as it does not synchronize the loop filter before returning a handle to the reconstructed frame in vpx_codec_get_preview_frame(), which can cause (false?) failures when running the test_reconstruct_buffer test. This may be related to a bug that does cause visible artifacts, which is also under investigation. This reverts commit 380d64ec. Change-Id: Iad710941e7731d44fc2bde63bc63d6763cc4629e