help: Always use xdg-open if available

Even if $DISPLAY is unset, xdg-open can be useful, and on systems that
have xdg-open, "open" is most likely some god awful outdated thing
called "openvt" elsewhere.

Fixes #6739

[ci skip]

share/functions/help.fish

@@ -68,8 +68,7 @@ function help --description 'Show help for the fish shell'
             if type -q cygstart
                 set fish_browser cygstart
                 # If xdg-open is available, just use that
-                # but only if an X session is running
-            else if type -q xdg-open; and set -q -x DISPLAY
+            else if type -q xdg-open
                 set fish_browser xdg-open
             end