Remove redundant unref of c->in in close_internal

The dav1d_data_unref_internal(&c->in) call in close_internal() is redundant because dav1d_flush() already calls dav1d_data_unref_internal(&c->in).

Note that when dav1d_open() calls close_internal() with flush=0 to free a newly constructed Dav1dContext, it is not necessary to unref c->in.