decodebin3: Don't send sticky events to unlinked decoder
This causes a lot of nasty side effects (like decoders assuming they are actually linked downstream).
The reason why this was done was to check whether a decoder could handle the actual caps, but this is the wrong way to do it.
The proper way to query whether a decoder can handle certain caps is via
GST_QUERY_ACCEPT_CAPS
which is already done just before.
Partially reverts !4677 (merged) and partially fixes #3160 (closed)