Remove the warning that `SnappyWriter` calls `Push(76490)` but may write less.
Remove the warning that `SnappyWriter` calls `Push(76490)` but may write
less.

This should be efficient for most `Writer`s (in particular `ChainWriter`
avoids wasteful blocks), and the expectation that `Push(min_length)` and
then writing less is efficient is now more widespread
(`FramedSnappyWriter` does that too).

PiperOrigin-RevId: 282524716

google/riegeli,google/riegeli,google/riegeli,google/riegeli