This commit removes the stepping stone analyzer. It has been deactivated
by default since at least Zeek 2.0, is dysfunctional in cluster settings
and has a bunch of other issued.
Relates to GH-1573
* All "Broxygen" usages have been replaced in
code, documentation, filenames, etc.
* Sphinx roles/directives like ":bro:see" are now ":zeek:see"
* The "--broxygen" command-line option is now "--zeexygen"