bluetooth: Don't free read_smoother in pa__done().
pa__done() calls stop_thread(), and stop_thread() already
frees the smoother. The duplicate freeing is not strictly
a bug, but static analyzers (in this case Coverity) may
complain about double-freeing, because when pa__done()
"frees" the smoother (which doesn't actually ever happen),
the pointer is not nulled. pa__done() then calls
bt_transport_release(), which will also free the smoother
if it's not NULL.
The analyzer complaint could be silenced also by nulling
the pointer in pa__done(), but since this is clearly
redundant code, I chose to remove it.