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