bug#25778: 25.1; [PATCH] Drastically simplify xdg-open check

[Eli Zaretskii <eliz@gnu.org>]

> From: Vasilij Schneidermann <v.schneidermann@gmail.com>
> Date: Fri, 17 Feb 2017 19:16:48 +0100
> 
> A user on #emacs brought it to my attention that the xdg-open detection
> for browse-url is more complex than it should be and fails detecting
> their Linux desktop.  Looking at the definition of
> `browse-url-can-use-xdg-open' reveals that it replicates the desktop
> environment check as done by `xdg-open` (which is done to detect the
> DE-specific default browser setting, not to prevent people not using a
> popular desktop environment from using xdg-open).  As enumerating all
> possible kinds of Linux desktop is a futile endeavor, I've completely
> eliminated this check from it so that the only ones left are whether
> we're in a graphical session (by checking $DISPLAY) and whether
> `xdg-open` has been found on $PATH.

First, checking $DISPLAY doesn't yet mean you are in a GUI frame.  We
have display-graphic-p for that.

And second, I don't understand what will happen with all the bugs
mentioned in the comments if we remove the code that was supposed to
avoid hitting them.  Maybe I'm missing something obvious.