gpm: unpin texinfo

It was a regression fixed a couple of years ago
https://lists.gnu.org/archive/html/bug-texinfo/2021-09/msg00012.html

pkgs/top-level/all-packages.nix

@@ -7690,7 +7690,7 @@ with pkgs;
   texinfoPackages = callPackages ../development/tools/misc/texinfo/packages.nix { };
   inherit (texinfoPackages)
     texinfo6_5 # needed for allegro
-    texinfo6_7 # needed for gpm and fwknop
+    texinfo6_7 # needed for fwknop
     texinfo6
     texinfo7
     ;
@@ -11085,10 +11085,6 @@ with pkgs;
 
   gpm = callPackage ../servers/gpm {
     withNcurses = false; # Keep curses disabled for lack of value
-
-    # latest 6.8 mysteriously fails to parse '@headings single':
-    #   https://lists.gnu.org/archive/html/bug-texinfo/2021-09/msg00011.html
-    texinfo = buildPackages.texinfo6_7;
   };
 
   gpm-ncurses = gpm.override { withNcurses = true; };