Topic branches are not in widespread use and the details on how are
managed are being reworked.
Drop the topic branches reference (now being renamed "collab" branches)
from the contribution guideline and just ask to open PRs against main,
as that's the common case right now, saves first time users from having
to dig through the various branches in the repository.
Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>
eec4b2336c