contint: drop jsduck from Debian slaves

We had ruby-jsduck as a Debian package which was used when we had
production slaves. Nowadays we run everything on labs slaves and can't
install jsduck straight from rubygems.org

Stop installing ruby-jsduck on Debian slaves. We still keep it for
Ubuntu though since Jenkins jobs have not been migrated yet.

Bug: T95008
Change-Id: If5a111447024b208e24d3363aa4f61deb2091e2c