stream: remove inline buffer optimization

Was probably worthless, and I can't measure a difference anymore (I used
to be able and it still seemed worth doing so back then).

When the default buffer size is enlarged in the next commit, the inline
buffer probably won't even be useful in theory, because the data will
rarely be on the same page as the other stream fields. It surely makes
the inline buffer seem like a ridiculous micro-optimization. Farewell...