FEC elements: document, remove irrelevant properties

FEC elements: document, remove irrelevant properties

The ulpfecenc "mux-seq" and "ssrc" properties were initially added
because the element did more than implement ULPFEC. As it was
decided that FLEXFEC would be implemented in a separate element,
both properties are now unneeded and confusing.

Change the default for the ulpfecenc multi-packet property,
as it is expected that most users of this element will be protecting video
streams.

Change the default property for the rtpredenc allow-no-red-blocks
property, as it should also be its default mode of operation.

https://bugzilla.gnome.org/show_bug.cgi?id=793843