decklinkaudiosrc/decklinkvideosrc: Do nothing in BaseSrc::negotiate() and...
decklinkaudiosrc/decklinkvideosrc: Do nothing in BaseSrc::negotiate() and always set caps in ::create()
We don't support negotiation with downstream but simply set caps based on the buffers we receive. This prevents renegotiation to other formats, and negotiation to NTSC in mode=auto in the beginning until the first buffer is received.
As side-effect of this, also remove various other caps handling code that was working around the behaviour of the default BaseSrc::negotiate().