thunderbird: remove package example that referred to old alias

This package no longer exists in nixpkgs; instead of bumping it, just
leave it unset.

Signed-off-by: Ihar Hrachyshka <[email protected]>

Author: booxter

modules/programs/thunderbird.nix

@@ -166,7 +166,6 @@ in {
         type = types.package;
         default = pkgs.thunderbird;
         defaultText = literalExpression "pkgs.thunderbird";
-        example = literalExpression "pkgs.thunderbird-91";
         description = "The Thunderbird package to use.";
       };