resampler: Drop pointless remix variable

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
asyncmsgq: Drop weird assert

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
pstream: Duplicate assignment, write.data is always NULL

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>