chore(jenkins): get rid of Opera from the Jenkins build script

The Opera launcher hasn't been installed for ages, but until Karma 1.4.0 the
error of Opera not being able to start was ignored. Karma has fixed the bug and
now Jenkins is failing.

This commit also removes Opera/Opera launcher mentions from the docs. We don't
support Opera officially anymore (it's sort-of supported via being based on
Blink).

Closes #15691