bug#26181: Remove etags

[Eli Zaretskii]

> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Tue, 21 Mar 2017 11:03:24 +0200
> 
> And [etags is] handy to have for now, at least until Universal Ctags
> is widely available across distributions

This indeed seems like the most important prerequisite, because the
command-line options accepted by Universal ctags are incompatible with
those of etags, so supporting both would be a pain.

> > Having our own tags program means we can tweak it to work better for our
> > own code; but conversely it would be better if our code Just Worked with
> > standard (ie external) tools.
> 
> The latter might mean learning to use the ctags output (Vim format).

You mean, if we want to support tools beyond Universal ctags?
(Because the latter supports Emacs-compatible TAGS output.)  Yes, if
we want to support other tools, we'd need to extend etags.el or rely
on external packages for parsing their tags tables.  E.g. GNU Global
doesn't support TAGS format even as an option, AFAIK.