Don't return poorly supported "special elements" in eglot-imenu

Don't return poorly supported "special elements" in eglot-imenu

Fix https://github.com/joaotavora/eglot/issues/758, https://github.com/joaotavora/eglot/issues/536, https://github.com/joaotavora/eglot/issues/535.

Eglot's eglot-imenu returned a structure compliant with the rules
outlined in imenu--index-alist.  In particular, it returned some
elements of the form

  (INDEX-NAME POSITION GOTO-FN ARGUMENTS...)

The original intention (mine) must have been to allow fancy
highlighting of the position navigated to with a custom GOTO-FN.

Not only was access to that fanciness never implemented, but many
other imenu frontends do not support such elements.

See for example https://github.com/joaotavora/eglot/issues/758, https://github.com/joaotavora/eglot/issues/536, https://github.com/joaotavora/eglot/issues/535.  And also related issues in other
packages:

https://github.com/IvanMalison/flimenu/issues/6
https://github.com/bmag/imenu-list/issues/58

So it's best to remove this problematic feature for now.  It can be
added back later.

* eglot.el (eglot-imenu): Simplify.

* NEWS.md: Mention change