graphparser: simplify condition in avfilter_graph_parse()

Since avfilter_graph_parse() creates the "[in]" inout for the first
unlabelled input pad, it is expected that it will create an "[out]"
inout for last unlabelled output pad, even in the case where it cannot
find any open input pad with that name.

This change removes the check on the existence of an open input pad
named "out", so it simplifies the checked condition while implementing
a more intuitive behavior.
