Now that all casper tests have been migrated to
puppeteer, there's no need for having casper
related things.
Removed the casperjs package and removed/replaced
casper in few places with puppeteer.
Only removed few of them which I'm confident
about. Also didn't make any changes in docs
as it would be easier to remove them while
adding puppeteer docs.
As of commit 87e72ac8e2 (#15267), we
need to be an owner for some of the tested functionality, not just an
administrator.
Signed-off-by: Anders Kaseorg <anders@zulip.com>