fish_command_not_found: remove legacy event handler

We no longer emit the "fish_command_not_found" event ourselves,
so the event handlers are only useful to users who
1. run "emit fish_command_not_found"
2. copy the definition of "fish_command_not_found" to their config and run
   that with fish < 3.2.

Probably no one does 1; there are no matches in
https://github.com/search?utf8=%E2%9C%93&q=%22emit+fish_command_not_found%22+language%3Afish&type=code

Reason 2 is less relevant after 5 years.

For additional evidence, none of our specializations
("/usr/libexec/pk-command-not-found" etc.)  react to the event,
and no one has ever complained.

Stop registering any fish_command_not_found as event handler,
for consistency and simplicity.

While at it, remove the documentation on how to make it work for
version < 3.2.

doc_src/cmds/fish_command_not_found.rst

@@ -50,25 +50,3 @@ Or the simple default handler::
     function fish_command_not_found
         __fish_default_command_not_found_handler $argv
     end
-
-Backwards compatibility
------------------------
-
-This command was introduced in fish 3.2.0. Previous versions of fish used the "fish_command_not_found" :ref:`event <event>` instead.
-
-To define a handler that works in older versions of fish as well, define it the old way::
-
-  function __fish_command_not_found_handler --on-event fish_command_not_found
-       echo COMMAND WAS NOT FOUND MY FRIEND $argv[1]
-  end
-
-in which case fish will define a ``fish_command_not_found`` that calls it,
-or define a wrapper::
-
-  function fish_command_not_found
-       echo "G'day mate, could not find your command: $argv"
-  end
-
-  function __fish_command_not_found_handler --on-event fish_command_not_found
-       fish_command_not_found $argv
-  end

share/config.fish

@@ -14,7 +14,7 @@ end
 if not status is-interactive
     # Hook up the default as the command_not_found handler
     # if we are not interactive to avoid custom handlers.
-    function fish_command_not_found --on-event fish_command_not_found
+    function fish_command_not_found
         __fish_default_command_not_found_handler $argv
     end
 end

share/functions/fish_command_not_found.fish

@@ -71,7 +71,7 @@ else if type -q pkgfile
     #     end
 else
     # Use standard fish command not found handler otherwise
-    function fish_command_not_found --on-event fish_command_not_found
+    function fish_command_not_found
         __fish_default_command_not_found_handler $argv
     end
 end