Remove ref-update trigger for rtd-merge template

This was originally added to help docs projects trigger a docs
update when a tag is deployed. Unfortunately ref-update appears to
trigger even for comments in Gerrit now. Not sure why but likely due
to Gerrit NoteDB. The Jenkins Gerrit Trigger configuration for
ref-update unfortunately does not provide any configuration for
specifying specific refs to trigger on.

Until there's an alternative it's better to disable this so that
Jenkins does not get flooded with queued jobs for Gerrit code
reviews that have active comments.

Issue: IT-18792
Signed-off-by: Thanh Ha <zxiiro@gmail.com>
Change-Id: I417e53d698e1454deb28b30d87e97147ce8d3038