opus_pvq: remove unneeded assert

Since the PVQ search has been well fuzzed and is guaranteed to never
break SUM(abs(y[])) == K, the assert is no longer needed.
Also the assert only prevented coding the wrong vector index but didn't
prevent crashes during searching for it, which made the assert rather
informational than practical.

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>
