This was broken by commit 1238137c.

When calling the generic coder with a max, it truncates the CDF, but
then tries to encode/decode with the Q15 entropy coder functions
with built-in adaptation. That causes assertion failures, because the
total probability isn't 32768.

We could fix it by re-scaling the CDF, and then doing adaptation
assuming there was no max, but that requires several special-case
code paths.

Instead, since non-robust streams were the only thing that still
required calling the generic coder with a max, and since the gain
from them is very small (and they require doing more DSP just to be
able to parse the stream), we simply remove the option and force
the use of robust streams all the time.

With robust streams enabled, encoder output should not change, and
all streams should remain decodable without decoder changes.

Thanks to Nathan Egge for reporting the problem.

Change-Id: I6c81481abb796688bf703d68f164d208e6a69f20