input: Remove p_playlist gc signaling now that the playlist is aware of vlc_InputSelectedStreamChanged.

This reverts commit 627aaa88.

Well, we do need to define the type first. Limitation of how we declared events.

input: No need to signal input's end to the playlist now that the playlist gets an event for that one.

(To better track when to attach/detach events).

Currently libvlc uses "state" callback to notice input state changes, and the playlist notice input state change via direct call from the input.
playlist can't use the "state" variable as it is also used by the playlist to ask the input to do a certain action.

The event approach is much more clear here.