playbin2: remove the group cond + cleanups

playbin2: remove the group cond + cleanups

Remove the group GCond that we used for waiting for groups to finish because we
use pad blocking on the selectors and counters instead for waiting for the
groups to complete.

remove the obsolete about_to_finish variable set while emiting the
about-to-finish signal and fix some old comments.

We don't need to take the playbin lock when querying the uridecodebin.