Do disown the completion updating without $last_pid

In this context, as it stands, $last_pid will give fish's pid (because
of pgroup shenanigans).

Since that doesn't really work, just `disown` without and let fish
figure out what the last process was.

Theoretically this has an issue if someone started a background
process *before* the python script *and* that exits before we run
disown.

That's a vanishingly small window and this is only run on first start,
so it seems acceptable.

Fixes #7739.

CHANGELOG.rst

@@ -766,7 +766,8 @@ Interactive improvements
 -  History newly imported from bash includes command lines using ``&&``
    or ``||``.
 -  The automatic generation of completions from manual pages is better
-   described in job and process listings (:issue:`6269`).
+   described in job and process listings, and no longer produces a
+   warning when exiting fish (:issue:`6269`).
 -  In private mode, setting ``$fish_greeting`` to an empty string before
    starting the private session will prevent the warning about history
    not being saved from being printed (:issue:`6299`).

share/functions/__fish_config_interactive.fish

@@ -94,7 +94,7 @@ function __fish_config_interactive -d "Initializations that should be performed
                 # Run python directly in the background and swallow all output
                 $python $update_args >/dev/null 2>&1 &
                 # Then disown the job so that it continues to run in case of an early exit (#6269)
-                disown $last_pid >/dev/null 2>&1
+                disown >/dev/null 2>&1
             end
         end
     end