MT#58535 rtpp_flags: remove unused `flags` bencode list item
Since the generic flags parsing is done (from now on) directly
in the `parse_rtpp_flags()`, there is no need to keep a separate
bencode list object to store them, which was originally intended
to be used later for parsing in `call_ng_flags_flags()`,
but not the case anymore.
Change-Id: I7898e29cabecfd6e8429c064d7b5904d94c50513